●CSS
#container {
display: grid;
grid-template-rows: 50px 200px;
grid-template-columns: 100px 1fr;
grid-template-areas:
"areaA areaA"
"areaB areaC";
}
#itemA {
grid-area: areaA;
background: #f88;
}
#itemB {
grid-area: areaB;
background: #8f8;
}
#itemC {
grid-area: areaC;
background: #88f;
}

長々とすみません。