ok so you have two sets of parenthesis and the second one is all subtracted from the first one
so what you do is since the entire thing is subtracted, you can get rid of the parenthesis through distribution of the minus sign, so you multiply each term by -1 in the second parenthesis
then you end up with 3x^5 + 6x^4 - 5x + 7 -6x^4 + 5x^3 - 7
and so now look at terms that have the same exponent. look at x^4, you are adding 6x^4 and you are subtracting 6x^4 and so if you add and subtract the same thing, its like adding zero, so they cancel each other. then you also have some constants doing the same thing so the 7's will cancel.
cleaning up what we now have is
3x^7 + 5x^3 - 5x
now look at the exponents. you have a degree of 7, 3, and 1. since none are equal, you cannot combine further.
therefore you are left with exactly that.