Step-by-step explanation:
The sum of the measures of the interior angles of a triangle is 180°. Then the sum of the given angles is 180°:
m∠P +m∠Q +m∠R = 180°
(5x)° +(5x)° +(8x)° = 180°
18x = 180 . . . . . . . . . . . . . . . collect terms, divide by °
x = 10 . . . . . . . . . . . . . . . . . . divide by 18. This is your desired result.