I'm pretty sure he means they did the problem by first figuring out the numbers a and b. That's the slow way. I can reveal the fast way if you want, but maybe you should think about it a bit more first! :)
I know the 'fast way'. Took me a second or two to get the answer, but it is so obvious that I cannot imagine anyone trying to solve this by calculating a and b first.
For novice students of algebra first learning to solve for values of variables, the idea of NOT solving for the values of the variables is a major step.
