A waiter had some customers. After w customers left he still had x customers. How many customers did he have at the start?
First, let's think about how many customers the waiter had before any left. We'll call that number $C$.

$C$ customers left, so now the waiter has $C-w$ customers.

But we know that after $w$ customers left, the waiter still had $x$ customers. So:

$C-w=x$

$C=x+w$

So the waiter had $x+w$ customers at the start.