A waiter had some customers. After w customers left he still had x customers. How many customers did he have at the start?

The waiter had w customers leave. This means that at some point he had w + x customers. 

We also know that after w customers left, he still had x customers. Therefore, the equation would be:

w + x = x

This equation says that w = 0. 

This means that the waiter had 0 customers leave and x customers at the start.