xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <EditText
        android:id="@+id/arg1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:ems="10"
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/arg2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:ems="10"
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="@+id/arg1"
        app:layout_constraintStart_toStartOf="@+id/arg1"
        app:layout_constraintTop_toBottomOf="@+id/arg1" />

    <Button
        android:id="@+id/add"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="onClickAdd"
        android:text="+"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/arg2" />

    <Button
        android:id="@+id/subtr"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="onClickMinus"
        android:text="-"
        app:layout_constraintEnd_toEndOf="@+id/add"
        app:layout_constraintStart_toStartOf="@+id/add"
        app:layout_constraintTop_toBottomOf="@+id/add" />

    <Button
        android:id="@+id/mul"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="onClickMul"
        android:text="*"
        app:layout_constraintEnd_toEndOf="@+id/subtr"
        app:layout_constraintStart_toStartOf="@+id/subtr"
        app:layout_constraintTop_toBottomOf="@+id/subtr" />

    <Button
        android:id="@+id/divide"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="onClickDel"
        android:text="/"
        app:layout_constraintEnd_toEndOf="@+id/mul"
        app:layout_constraintStart_toStartOf="@+id/mul"
        app:layout_constraintTop_toBottomOf="@+id/mul" />

    <TextView
        android:id="@+id/answer"
        android:layout_width="153dp"
        android:layout_height="118dp"
        app:layout_constraintEnd_toEndOf="@+id/divide"
        app:layout_constraintStart_toStartOf="@+id/divide"
        app:layout_constraintTop_toBottomOf="@+id/divide" />

</androidx.constraintlayout.widget.ConstraintLayout>