Since ∠VST measures 90°, the measure of ∠RSV must also be 90° because the two angles are linear pair.
Since m∠RSU=51°, then the measure of ∠USV is as follows:
[tex]\begin{gathered} m\angle USV=m\angle RSV-m\angle RSU \\ m\angle USV=90\degree-51\degree \\ m\angle USV=39\degree \end{gathered}[/tex]