[tex] \bf \begin{cases}\stackrel{\measuredangle TRS}{2x+5y}=180\\2x=180-5y\\\boxed{x}=\frac{180-5y}{2}\\[-0.5em]\hrulefill\\\stackrel{\measuredangle XRS\cong \measuredangle TRX}{3x+3y}=90\end{cases}~\hspace{7em}\stackrel{\textit{using \boxed{x} in the second equation}}{3\left( \boxed{\frac{180-5y}{2}} \right)+3y=90} [/tex]
[tex] \bf \cfrac{540-15y}{2}+3y=90\implies \stackrel{\textit{multiplying both sides by }\stackrel{LCD}{2}}{540-15y+6y=180}\implies -9y=-360\\\\\\y=\cfrac{-360}{-9}\implies \blacktriangleright y=40 \blacktriangleleft \\\\\\\stackrel{\textit{using \underline{y} in the first equation}}{2x+5(40)=180}\implies 2x+200=180\implies 2x=-20\\\\\\x=\cfrac{-20}{2}\implies \blacktriangleright x=-10\blacktriangleleft [/tex]
if you ever wonder why we multiplied by the LCD of 2, is just to do away with the denominators.
