Use both the Chain and Product Rule.

Before you differentiate, you could solve for $y$.
Or, you could use implicit differentiation:

$\frac{d}{dx}(3xy-x^{2})=\frac{d}{dx}(2)$

$3y\left (\frac{dx}{dx} \right )+3x\left ( \frac{dy}{dx} \right )-2x\left ( \frac{dx}{dx} \right )=0$

$\text{Note: }\left ( \frac{dx}{dx} \right )=1$

$\text{Solve for }\left ( \frac{dy}{dx} \right ).$

Solve for $y$ first.