There are two ways to solve part b. The first method uses the formula for a kite but gives a complicated answer. The second method uses the observation that triangles OPR and OQR are congruent to each other and are both right-angled triangle. Method 2 is the faster method and gives a cleaner answer. Method 2 is also probably the suggested method considering the given numbers i.e. 4/alpha and 5/alpha is probably a hint to make use of the right-angled triangle.