Applying the segment addition postulate, SU = 10
Based on the segment addition postulate, when a point lies in between two endpoints on a line segment, the sum of the lengths of the two smaller segments equals the length of the longer segment.
Given:
SU = 2x - 12
TU = 6
ST = x - 7
Thus:
TU + ST = SU
6 + x - 7 = 2x - 12
-1 + x = 2x - 12
-2x + x = 1 - 12
-x = -11
x = 11
SU = 6 + x - 7
SU = 6 + 11 - 7
SU = 10
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