Answer :
[tex] \left \{ {{2a+3b=12} \atop {5a-b=13\ \ | *3}} \right. \\\\ \left \{ {{2a+3b=12} \atop {15a-3b=39}} \right. \\+-----\\ add\ both\ equations\\\\
17a=51\ \ \ | divide\ by\ 17\\\\
a=3\\\\
b=5a-13\\\\
b=5*3-13=15-13=2\\\\
b=2[/tex]
