I thought I know this one but the answer stumps me:

(3a - 3b) / (6a - 6b)

Answer key: 1/2

After cancelling out the "a" and "b", I would think I have (3 - 3) in the numerator and (6 - 6) in the denominator and the answer would be 0. How did the answer turns out to 1/2.

I think I'm missing some concept here. Pls help. Thanks.