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
