In this question, the triangle has 2 same side:SR=ST. That mean the angle of R will be same as T.
S= 7x-54
If STU=8x then T=R= 180-8x
If you add all 3 angle of triangle it will be 180. Then,
R+S+T= 180
7x-54 + (180-8x)+(180-8x)= 180
7x +360-16x= 180+54
-9x= 234-360
x= 14
