[tex]\boxed{\begin{array}{l}\tt a)\\+\underline{\begin{cases}\sf x-\diagdown\!\!\!\!\!\!3y=9\\\sf 2x+\diagdown\!\!\!\!\!\!3y=6\end{cases}} \\\sf3x=15\\\sf x=\dfrac{15}{3}\\\sf x=5\\\sf x-3y=9\\\sf 5-3y=9\\\sf 3y=5-9\\\sf 3y=-4\\\sf y=-\dfrac{4}{3}\\\sf S=\bigg(\bigg\{5,-\dfrac{4}{3}\bigg\}\bigg)\end{array}}[/tex]
[tex]\boxed{\begin{array}{l}\tt b)\\\begin{cases}\sf3x+2y=5\cdot3\\\sf5x-3y=2\cdot(2)\end{cases}\\+\underline{\begin{cases}\sf9x+\diagdown\!\!\!\!\!\!6y=15\\\sf10x-\diagdown\!\!\!\!\!\!6y=4\end{cases}} \\\sf19x=19\\\sf x=\dfrac{19}{19}\\\sf x=1\\\sf 3x+2y=5\\\sf 3\cdot1+2y=5\\\sf 3+2y=5\\\sf2y=5-3\\\sf 2y=2\\\sf y=\dfrac{2}{2}\\\sf y=1\\\sf S=(\{1,1\}) \end{array}}[/tex]
