-----------------------------------------------------------------------------
Isosceles triangle
-----------------------------------------------------------------------------
∠STR = ∠SRT = 5x - 30
-----------------------------------------------------------------------------
Adjacent angles on a straight line is 180°
-----------------------------------------------------------------------------
∠STR + ∠ STU = 180°
(5x - 30) + 5x = 180
5x - 30 + 5x = 180
10x - 30 = 180
10x = 180 + 30
10x = 210
x = 21
-----------------------------------------------------------------------------
Answer: x = 21
-----------------------------------------------------------------------------