A waiter had some customers. After w customers left he still had x customers. How many customers did he have at the start?
Suppose that y customers left after x customers. Then we have the following equation:

y = w - x

Solve for x:

x = w - y

Now we know that the waiter had w + x customers at the start. Therefore, we can substitute x with w - y:

w + x = w + w - y = 2w - y

We know that y customers left, so:

2w - y = 2w - (w - x) = 2w - w + x = w + x

Therefore, we can conclude that w + x = w + x, which means that x = 0. 

The waiter had w + 0 customers at the start, which means that he had w customers at the start.