If $(a+2x) (bx+3) = 3x^2 +cx+12$ for all values of $x$. What are the two possible values of $c$ ?

Question

$\begin{array}{1 1} (a)\; c= 6 \\ (b)\; c=8 \\ (c)\; c=12 \\ (d)\; c=15 \end{array}$