Compute forward

\begin{align*} a & = x + y = 1 + 2 = 3 \\ b & = y + 3 = 2 + 3 = 5 \\ c & = ab = 3 \cdot 5 = 15 \end{align*}

getting

\begin{equation*} f(x) = c = 15. \end{equation*}