In this question, we are required to change the subject of the formulae from b to a.
Our approach is to successfully manipulate the variables so as to have a on the one side isolated.
We have:
[tex]\begin{gathered} b=9+\frac{1}{3}qa \\ We\text{ subtract 9 from both sides to get:} \\ b-9=\frac{1}{3}qa \\ \text{Next we multiply both sides by }\frac{3}{q}\text{ to get} \\ \frac{3}{q}(b-9)=a \end{gathered}[/tex]This gives us a as:
[tex]a=\frac{3}{q}(b-9)[/tex]