.flex-container
  .block

.flex-container {
  display: flex;
  justify-content: space-between;
  width: 400px;
  height: 400px;
  border: 2px solid black;
  background-color: #dddddd;
}

.block {
  width: 200px;
  height: 200px;
  background-color: pink;
  border: 1px solid black;
  margin: auto;
}