So you reversed the subtraction a bit; the equation you're gonna be solving is [tex]3x^2+8x-7-(5x^2+2x-11)[/tex] .
First step ya gonna do is to reverse the + and - symbols in the trinomial thats in the parentheses so your equation looks like [tex]3x^2+8x-7-5x^2-2x+11[/tex]
Now just combine like terms and your answer should be [tex]-2x^2+6x+4[/tex]