STU + STR = 180
8x + STR = 180 [ STU = 8x ]
STR = 180 - 8x
STR = SRT [ RS ≈ ST ]
Now, In ΔSRT,
we have, RST = 7x - 54, STR = SRT = 180-8x
Their sum is 180, as it constitutes a triangle.
7x-54 + 2(180-8x) = 180
7x - 54 + 360 - 16x = 180
306 - 9x = 180
9x = 306 - 180
9x = 306 - 180
x = 126/9
x = 14
So, option A is your answer.
Hope this helps!
