recall that
sin(x) = cos(90-x)
therefore
cos((2/3)x + 20) = cos(90 - [2x - 10])
(2/3)x + 20 = 100 - 2x
2x + (2/3)x = 80
divide through by 2
x + (1/3)x = 40
(4/3)x =40
4x = 40*3
4x = 120
x = 30
since it is a right angled triangle, the angles are 30, 60 and 90
