[deepspace]

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.

[lupire]

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.

[dhosek]

Or for high school teachers of algebra as it turned out.