<div class='container'>
    <div class='navigation'>Navigation Here</div>
    <div class='main-content'>Main Content Here</div>
</div>

.navigation { 
  height: 500px;
  width: 300px;
  margin-right: 30px;
  float: left;
  background: red;
}

.main-content{
  width: calc(100% - 330px);
  width: -moz-calc(100% - 330px);
  width: -webkit-calc(100% - 330px);
  height: 500px;
  float: left;
  background: blue;
}