how to use float left and right in flex code example

Example 1: flex force div right side

.parent {
  display: flex;
}
.child {
  margin-left: auto;
  order: 2;
}

Example 2: flexbox align right and left

.primary-nav {
    display:-webkit-flex;
    display:flex;
    list-style-type:none;
    padding:0;
    justify-content:flex-end;
}

.left {
    margin-right:auto;
}