I assume the way it's written is like this
4x^2 - 8x + 4 - (2x^2 - 3x + 7)
If that's the case, just combine like terms.
Let's begin with 4x^2. It has the same components as the -2x^2 (remember, since it's being subtracted in the original equation, the number is negative.)
4x^2 - 2x^2 = 2x^2
Now -8x and 3x (be careful, sometimes there's a minus sign in and out of the parentheses, this makes the number positive again
-8x + 3x = -5x
And then 4 and -7
4 - 7 = -3
So the simplified version is finally:
2x^2 - 5x - 3
Hope this helped! If you need me to explain anything, don't be afraid to ask.