8. If RST is isosceles, first solve for x, then find m S.

it is given that,
the triangle is isosceles triangle,
so, the two opposite angle will be the same,
m15x - 31 = 9x + 11
15x - 9x = 31 + 11
6x = 42
x = 42/6
x = 7
thus,the answer is x = 7
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