If ax - 4y = -6 has a slope of - 3/2. What is the value of a?

Answer: Option C

$$\begin{array}{rl}& ax-4y=-6\\ & \Rightarrow 4y=ax+6\end{array}$$

$$\begin{array}{rl}& y=\frac{a}{4}x+\frac{6}{4}\\ & \frac{a}{4}=\frac{-3}{2}\\ & \boxed{a=-6}\end{array}$$