looks like you did just fine.. .the only part is the sign for the slope, is all
[tex]\bf \textit{slope of }\overline{QR}\implies \cfrac{-2b}{2c-2a}\implies \cfrac{-b}{c-a}\iff \cfrac{b}{a-c}
\\\\\\
\textit{slope of }\overline{ST}\implies \cfrac{0-b}{c-a}\implies \cfrac{-b}{c-a}\iff \cfrac{b}{a-c}[/tex]
