html1

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <link rel="stylesheet" href="style.css">
  <title>Document</title>
</head>

<body onload="start()">
  <div id="app-main">
      <header class="app-header">
          <h1>header</h1>
        </header>

        <section class="app-container">
          <div class="app-container__box">
            <ul class="app-container__links">
              <li class="app-container__links__item"><a href="#">link</a></li>
              <li class="app-container__links__item"><a href="#">ax2+ bx + c = 0</a></li>
              <li class="app-container__links__item"><a href="#">link3</a></li>
            </ul>
          </div>

          <embed src="center.html" width="100%" min-height="900px;" >

          <div class="app-container__box app-container__box--news">
            <div class="app-container__box__title">
              <h2>news</h2>
            </div>
            <div class="app-container__box__context">
                <p>Cześć <span id="name-span"></span></p>
                <div id="day"></div>
                <div id="timer"></div>
                <p>Przedmiot: <span id="subject-span"></span></p>
                <p>Urodziny za: <span id="birthday-span"></span></p>


            </div>
          </div>
        </section>

        <footer class="app-footer">
            <span>&copy; Copyright 2019 </span>
        </footer>
  </div>

  <script src="script.js"></script>
</body>
</html>


<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <link rel="stylesheet" href="style.css">
  <title>content</title>
</head>
<body  style="overflow: hidden">

  <div class="app-container__content">
    <h1>Podaj dane:</h1>

      <input type="text" id="first" placeholder="A" style="float:left; width:30%">
      <input type="text" id="second" placeholder="B" style="float:left; width:30%">
      <input type="text" id="third" placeholder="C" style="float:left; width:30%" >
      <input type="submit" value="OBLICZ" onclick="matma()" style="width:30%; ">

      <div id="wyniki">


      </div>

  </div>

  <script src="script.js"></script>
</body>
</html>