.block_left {
  background-color: red;
  height: 70px;
  width: 100px;
  float: left;
  border-right: 2px solid blue;
  border-bottom: 2px solid blue;
}

.block_right {
  background-color: red;
  height: 70px;
  width: 100px;
  float: right;
  border-left: 2px solid blue;
  border-bottom: 2px solid blue;
}

.div2 {
  background-color: white;
  font-size: 1.5em;
  border: 2px solid blue;
}

.parent {
  height: 300px;
  width: 500px;
}

<div class="parent">
  <div class="block_left"></div>
  <div class="block_right"></div>
  <div class="div2" contenteditable="true">
    "Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut..."
  </div>
</div>