1. The problem is to find an angle in a triangle using the sine rule. 2. The sine rule formula is $$\frac{a}{\sin A} = \frac{b}{\sin B} = \frac{c}{\sin C}$$ where $a$, $b$, $c$ are side lengths and $A$, $B$, $C$ are the opposite angles. 3. To find an angle, rearrange the formula to solve for the sine of the angle: $$\sin A = \frac{a \sin B}{b}$$ if you know sides $a$, $b$ and angle $B$. 4. Then, find the angle $A$ by taking the inverse sine (arcsin): $$A = \arcsin\left(\frac{a \sin B}{b}\right)$$. 5. Important: The value inside arcsin must be between -1 and 1 for a valid angle. 6. This method allows you to find the measure of an unknown angle when you know two sides and one angle.