Please help with these two problems! Need to use the equation to predict the skid distance at 50 and 100 mph.

Can someone me out here number 4 plz

Which expression is equivalent to (5x^7+7x^8) − (4x^4−2x^8)? a. 9x^8 + 5x^7−4x^4 b. 3x^8 + 7x^7 c. 12x^8 − 2x^4 d. 5x^8 + 5x^7 + 4x^4

What is the answer to this equation?