Answer :

mariamikayla
[tex]3a+b=54 \\\\ 3a+9=54 \\\\ 3a=54-9 \\\\ 3a= 45 \\\\ \boxed{a=\frac{45}{3}\to15}[/tex]
luana
[tex] \left \{ {{3a+b=54} \atop {b=9}} \right. \\\\Substitution\\\\ 3a+9=54\ \ \ | subtract\ 9\\\\3a=45\ \ \ | divide\ by\ 3\\\\ a=15\\\\ b=9[/tex]