How do you solve for functions

For example

f (x) = 3x+2 and f (x) = x/x-1

The equation for a function is

f (x) = x + b

So consider this example function:

f (4) = 3x + 12

We know that x = 4 so we would substitute 4 for x.

f (4) = 3 (4) + 12

f (4) = 12 + 12

f (4) = 24

Since f (x) is equivalent to y, you could also say;

f (4) = y

y = 24