Factoring by grouping. 3x-3y-ax + ay Step 1. (3x - 3y) + (-ax + ay) = 3(x-y) + a (-x+y) Step 2. Factor out -a. 3(x-y) - a(x-y) Step 3. (X-y)(3-a) Im stuck on Step Two. The book doesn't explain How they were able to change the positive sign inside the second parenthesis to a negative sign. Also, how were they able to change the sign in front of the variable "a" from a positive In step 1 to a negative in step 2?