You have the following algebraic expression for the given segments:
SU = 16x - 1
ST = 9
TU = 5x + 1
In order to find the values of x, you consider that SU = ST + TU. You replace the algebraic expressions for the segments, and dolve for x, just as follow:
SU = ST + TU
(16x - 1) = (9) + (5x + 1) eliminate parenthesis
16x - 1 = 9 + 5x + 1 simplify similar terms right side
16x - 1 = 9 + 1 + 5x
16x - 1 = 10 + 5x subtract 5x both sides, and add 1 both sides
16x - 5x = 10 + 1 simplify similar terms
11x = 11 divide by 11 both sides
x = 11/11
x = 1
Hence, the solution for x is x = 1