Factorize x²+7x+12 ===> (x+3)(x+4)
Factorize x²-3x-28 ===> (x-7)(x+4)
4x + 12 can be written====> 4(x+3)
Then the question could be written as follows:
[(x+3)(x+4) / (x-7)(x+4) ] / 4(x+3) = [(x+3)(x+4) / (x-7)(x+4) ].[4(x+30]
After simplification you get 1 / 4(x-7)