Explanation:
In the figure above, q^o = 30^o (vertically opposite angles)

(P + 2q)^o + 30^o = 180^o(angles on a straight line)

p + 2 x 30^o + 30^o = 180^o

p + 60^o + 30^o = 180^o

p + 90^o = 180^o

p = 180^o - 90^o

= 90^o