<div class="main"></div>
<div class="box-1"></div>
<div class="box-2"></div>

body {
  height: 100vh;
  margin: 0;
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  grid-template-rows: repeat(2, 1fr);
}

.main {
  grid-column: 1 / 3;
  grid-row: 1 / 3;
  background: tomato;
}

.box-1 {
  background: royalblue;
}

.box-2 {
  background: seagreen;
}