Step-by-step explanation:
this is a nice parallelogram.
that means it has 2 pairs of equal and parallel sides (opposite of each other), 2 pairs of equal angles (also opposite of each other).
as in every quadrilateral the sum of all angles is 360°.
and because of the 2 pairs of equal (yet opposite) angles, every angle combines with one of its neighbors to 180°.
so,
(3x + 5) + (8x - 12) = 180
3x + 5 + 8x - 12 = 180
11x - 7 = 180
11x = 187
x = 17
and then the angle P is
3x + 5 = 3×17 + 5 = 51 + 5 = 56°
