The line at the top signifies 180 degrees, as does any straight line. The 150° that intersects with P tells you that P must be 30 degrees, as 180 - 150 = 30.
The three angles of a triangle must always equal 180°. Angle R tells you that it’s 90°, shown by the square instead of a curve. If you subtract the value of P we found before, and the value of R we just found, you get your answer.
