If $3x+4 =y$ and $y=cx+5$ are equations of perpendicular lines, then what is the value of $c$ ?

Question

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