MATH help; what am I doing wrong? I have shown my work for a) and I don't know how to do this obviously?

You started it correctly. You just have to simplify it. As previously mentioned, two negatives make a positive so the result of:

f(0) = 12/5

And then for the others, just substitute the "x" in again and simplify again.

I'll show you the second from last to show you what you need to do there as you should get the previous ones just fine on what you know from the first one:

f(x) = (x - 12) / (13x - 5)

Solve for:

f(x + 5)

Like the others, whatever is inside the function gets substituted instead of the "x" and you simplify, so we get:

f(x + 5) = (x + 5 - 12) / [13(x + 5) - 5]

Simplify this to get:

f(x + 5) = (x - 7) / (13x + 65 - 5)
f(x + 5) = (x - 7) / (13x + 60)

Since you had a variable in the input the output will be in terms of that variable. This is as simplified as you can go.

The last one will be similar, just with two variables instead of the one.

Hope this helped. Please give best answer if it did. Thanks.

Your steps are correct, but you have failed to finalize the answer by dividing the numerater by the denominator.

(a)
f(0) = [(0) - 12]/[13(0) - 5] = (-12)/(-5) = 2.4

are you retarded? negative over negative is positive.