∠STU and ∠STR are supplementary, therefore:
[tex]\begin{gathered} m\angle STR+m\angle STU=180 \\ where \\ m\angle STR\cong m\angle SRT=30 \\ m\angle STU=3x \end{gathered}[/tex][tex]\begin{gathered} so\colon \\ 30+3x=180 \\ 3x=180-30 \\ 3x=150 \\ x=\frac{150}{3} \\ x=50 \end{gathered}[/tex]
