Question

Factor completely. 4(x-2)^2 + 5 (x-2) (y+4) - 21 (y+4)^2

Answer 1

https://www.google.com/search?q=4(x-2)%5E2+%2B+5+(x-2)+(y%2B4)+-+21+(y%2B4)%5E2&oq=4(x-2)%5E2+%2B+5+(x-2)+(y%2B4)+-+21+(y%2B4)%5E2&aqs=chrome..69i57&sourceid=chrome&es_sm=0&ie=UTF-8 I think this will help.

Answer 2

well if you want to factor you must first expand the simplified version:
4(x - 2)(x - 2) + 5(x - 2)(y + 4) - 21(y + 4)(y + 4)

Answer 3

\(\normalsize\color{blue}{(4x-7y-36)(x+3y+10) }\)
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http://www.wolframalpha.com/input/?i=4%28x-2%29%5E2+%2B+5+%28x-2%29+%28y%2B4%29+-+21+%28y%2B4%29%5E2+++factor