RS = ST
so
m<SRT = m<STR.
as you know sum of interior angles = 180
so
m<RST + m<SRT + m<STR = 180
but m<RST = 7x - 54 and m<STU = 8x
let unknown angle (m<SRT = m<STR) = y
so now you have
m<RST + m<SRT + m<STR = 180
7x - 54 + y + y= 180
7x - 54 + 2y = 180 (1)
and
m<STU + m<STR = 180
8x + y = 180 so y = 180 - 8x
substitute y = 180 - 8x into equation (1) 7x - 54 + 2y = 180
7x - 54 + 2y = 180
7x - 54 + 2(180 - 8x)= 180
7x - 54 + 360 - 16x = 180
-9x + 306 = 180
-9x = -126
x = 14
answer
B. x = 14
