Step-by-step explanation:
she has created 2 similar triangles, as we have 2 equal sides, and their enclosed angle is also the same due to the rules of angles of crossing lines.
so, SAS is given and that proves that the 2 triangles are similar.
now, as the corresponding side lengths are equal, it also means that VT is equal to RU.
so, measuring VT gives us also the length of RU.
we could also measure the actual angle (either RSU or VST) and then work with the enhanced Pythagoras :
c² = a² + b² - 2ab×cos(S)
with c being the baseline opposite of the angle at S.