Hey!
So (f-g)(x) just means f(x) - g(x). So f(x) would come first, so it would look like (4^x-1 + 6) - (2x-5).
Distribute the minus sign and it would come out to be 4^x-1 + 6 - 2x + 5
Then combine like terms: 4^x-1 - 2x +11.
Your answer is A!
Hope this helps!
