The answer is a i just need working it out

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!