from tkinter import *
import random

root = Tk()

c = Canvas(root, width = 400, height = 20)
c.create_text(200, 10, text= "Введите пример : ")
c.pack()

e = Entry(root, width = 20)
e.pack()

b = Button (root, width = 20, text = "Решить")
b.pack()

l = Label(root, width = 50, bg = "Black", fg = "White")
l.pack()

def main(event):
    p = e.get()
    p = str(p)
    h = eval(p)
    h = str(h)
    l["text"] = ''.join(h)

b.bind ("<Button-1>", main)

root.mainloop()