Answer:
x = 3
Step-by-step explanation:
The sum of interior angles in a triangle measure to 180°.
So the sum of angles = 180°
(21x - 3) + (17x + 9) + (15x + 15) = 180
21x - 3 + 17x + 9 + 15x + 15= 180
21x + 17x + 15x -3 + 9 + 15 = 180
53x + 21 = 180
53x = 180 - 21
53x = 159
x = 159/53 = 3
