A flash unit has a capacitor of 150 µF charged to 200 V…

Step 1. The charge. $Q = CV = 150 \times 10^{-6}\ \text{F} \times 200\ \text{V} = 0.030\ \text{C} = 30\ \text{mC}$.

Step 2. The energy. $U = \tfrac{1}{2}CV^2 = \tfrac{1}{2} \times 150 \times 10^{-6}\ \text{F} \times (200\ \text{V})^2 = 3.0\ \text{J}$.

Step 3. The power. $P = \dfrac{U}{t} = \dfrac{3.0\ \text{J}}{2.5 \times 10^{-3}\ \text{s}} = 1200\ \text{W}$.