f(0) = 0 + 3 = 3

f(-4) = -4 + 3 = -1

f(-12) = -12 + 3 = -9

f(12) = 12 + 3 = 15

f(x + 8) = x+ 8 +3 = x + 11

In your examples, notice that there are different values in parentheses. You are given different inputs and are asked to find the corresponding outputs.

f(0) means that we want to know what happens when we input 0 as the x-value.

f(x) = x+3

f(0) = 0+3 = 3

You would solve f(-4), f(-12), and f(12) the same way. Just substitute the value in the parentheses wherever there is an x in the original equation.

f(x+8) is a bit trickier, but its the same concept. The input is now x+8 instead of x. So now we have to replace x with (x+8).

f(x) = x+3

f(x+8) = (x+8) + 3 = x+11