Assuming that the two angles under 5a/4 are 90°:
(Is that given in the assignment?? if line AB is horizontal they must be 90° as gravity pulls in the perpendicular direction)
theta(A) = 180° - 90° - B
angle opposite of theta(P) = 180° - 90° - B
angle opposite of theta(P) = theta(P)
therefore:
angle opposite of theta = theta(A)
theta(A) = theta(P)