[tex]Answer: \\ Since \: \widehat{SRT} = \widehat{STR} \: \\and \: \widehat{RTU} = \widehat{STR} + \widehat{STU} = 180^{\circ} \\ \Rightarrow \widehat{SRT} + \widehat{STU} = 180^{\circ} \\ \Leftrightarrow 4x - 20 + 6x = 180 \\ \Leftrightarrow10x = 200 \\ \Leftrightarrow x = 20 \\ \widehat{SRT} = 4 \times 20 - 20 = 60^{\circ}[/tex]
