angle = 90-x
complement of angle = x
supplement of angle = x+90
x^2 = 3(x+90)-90
x^2 = 3x+270-90
x^2 =3x+180
x^2-3x-180=0
(x+12) (x-15)=0
x=-12 x=15
cannot have negative angle so x=15
angle = 90-15 = 75
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