<div class='main'>
  <div class='main-left'></div>
  <div class='main-right'></div>
</div>

body {
  margin: 0;
  padding: 0;
    
  widht: 100%;
  height: 100%;
}
.main {
  width: 880px;
  height: 500px;
  
  display: flex;
  
  background: #000;
  
  &-left {
    width: 100%;
    min-width: 400px;
    
    background: pink;
  }
  
  &-right {
    // display: none;
    margin-left: 30px;
    width: 450px;
    
    background: green;
  }
  
}