Question

What is the equation for the measure of Angle P?

Answer 1

There is a theorem for inscribed quadrilaterals in a circle. This theorem states that: "Opposite angles in any quadrilateral inscribed in a circle are supplements of each other".
In other words, the sum of each two opposite angles in a quadrilateral inscribed in a circle is 180.

Based on the above:
measure angle O + measure angle Q = 180
measure angle R + measure angle P = 189

Therefore:
measure angle P = 180 - measure angle R