Question

If f(x) = x + 5, what does f(8) represent?

Answer 1

All you do is plug in 8 for x. 

So it f(8) = 8 + 5
which equals 13 

Hope this helps!

Answer 2

When you see f(a) it literally means to use a in place of the variable in the function.

If f(x)=x+5

f(8)=8+5

f(8)=13