Answer:
The explanation Marc can give
Jose is, you should have added 12 and –12 for the xy terms
Explanation:
Josh calculation
-8x^2y-17xy^2+12xy-8+5x^2y-xy^2-12xy=-3x^2y-18xy^2+24xy-8
Correct calculation
-8x^2y-17xy^2+12xy-8+5x^2y-xy^2-12xy
Collect like terms
= -8x^2y+5x^2y-17xy^2-xy^2+12xy-12xy-8
= -3x^2y-18xy^2-8
The explanation Marc can give
Jose is, you should have added 12 and –12 for the xy terms.