3x^2-12x+7
Explanation:
In this sort of question you collect like terms. The first thing I recommend you do is open up the brackets by multiplying everything inside the bracket by -1.
This will chance the (2x^2+7x-4) to -2x^2-7x+4.
Then you can collect like terms
5x^2 - 2x^2 = 3x^2
-5x-7x = -12x
3 - -4 =7
overall you get 3x^2-12x+7 which is option A