in dispay flex how some item starts from left right code example

Example 1: 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;
}

Example 2: flex item right

header > div:nth-child(3) {    margin-left: auto;}