$(sx+3)(tx-3)=4x^2+ux-9$ for all the values of $x$ and $s+t=4$, what are the possible values of $u$ ?

Question

$\begin{array}{1 1} (a)\; 0 \\ (b)\; 1 \\ (c)\; 2 \\ (d)\; 3 \end{array}$