Explanation:
well, the same principle as for a).
just instead of sine we need to use cosine here.
remember the trigonometric triangle : the up/down leg is sine (multiplied by the radius of the surrounding circle, which is the Hypotenuse of the triangle). and the left/right leg is cosine (multiplied by the Hypotenuse).
so,
12 = 15×cos(x)
cos(x) = 12/15 = 4/5 = 0.8
x = 36.86989765...°
of course, if you prefer, you could use Pythagoras
c² = a² + b²
to get the missing 3rd side, and then use that with sine the same way as for a).