Hello,
[tex]9a= 12a+3\dfrac{b}{4} \text{ *** subtract 12a ***}\\ \\<=> -3a= 3\dfrac{b}{4} \text{ *** divide by 3 ***}\\ \\<=> -a= \dfrac{b}{4} \text{ *** multiply by 4 ***}\\ \\<=>\Large \boxed{\sf \bf b=-4a}[/tex]
Thank you
