m<STR = 180 - 8x
m<SRT =m<STR = 180 - 8x by given
m<STU = m<RST + m<SRT by exterior angle equals the sum of the opposite interor angles
8x = (7x - 54) + (180 - 8x) regroup common terms together
8x = 7x - 8x -54 + 180
8x = -x + 126 combine common terms
9x = 126
x = 126/9
x = 14 this is a. 14