The first thing you need to do is to distribute the - throughout the second expression, that will let you get rid of the parenthesis. Then combine the like terms.
(4x2+8xy+2y2)-(−9x2 − 4xy − 7y2)
4x2+8xy+2y2+9x2 + 4xy + 7y2
13x2+12xy+9y2
Your answer is C.