#5 is very nicely and correctly done.
#7 says: "No matter what X may be, this function of it is always 9 more than 1/2 of X .".
That's a very powerful statement. Now you know that if X is ever 2, the function will be 1/2(2)+9 which is 10.
If X is ever zero, the function will be 1/2(0)+9 which is 9. If X is ever a cow, the function will be 1/2 of a cow, plus the number 9. Which makes no sense, but that's what the function says.
So, when X is -8, the function is 1/2 of -8, plus 9. Which is 5 ... the 'f' of -8.
Whatever X happens to be at the moment, just write that number in place of X in the function, and it'll show you the function of what X is.
f(a bazillion) = 1/2(a bazillion) + 9 .
f(a-28) = 1/2(a-28) + 9 (but simplify it)