[tex]\displaystyle \\
\frac{12+3b}{18+2a} = 0,\!\!(6) \\ \\
\frac{12+3b}{18+2a} = \frac{6}{9} \\ \\
\frac{12+3b}{18+2a} = \frac{2}{3} \\ \\
2(18+2a)=3(12+3b) \\ \\
\underline{36}+4a = \underline{36} + 9b \\ \\
4a= 9b \\ \\
\frac{4a}{b} =9 \\ \\
\bold{\frac{a}{b} }= \boxed{\bold{\frac{9}{4} }}[/tex]