H - 3q = gr
make g the subject of the relation.
H - 3q = gr
divide through by r
[tex]\begin{gathered} H\text{ - 3q = gr} \\ \frac{H}{r}\text{ - }\frac{3q}{r}\text{ = g} \end{gathered}[/tex]Final answer
[tex]g\text{ = }\frac{H}{r}\text{ - }\frac{3q}{r}\text{ or g = }\frac{H\text{ - 3q}}{r}[/tex]