Given that 'M' varies directly with the square of 'd',
[tex]M\propto d^2[/tex]Given that 'M' varies inversely with the square root of 'x',
[tex]M\propto\frac{1}{\sqrt[]{x}}[/tex]Combining the relationships,
[tex]M\propto\frac{d^2}{\sqrt[]{x}}[/tex]Let 'k' be the constant of proportionality. Then,
[tex]M=k\cdot\frac{d^2}{\sqrt[]{x}}[/tex]Given that M=12 when d=3 and x=4,
[tex]\begin{gathered} 12=k\cdot\frac{(3)^2}{\sqrt[]{4}} \\ 12=k\cdot\frac{9}{2} \\ k=\frac{12\cdot2}{9} \\ k=\frac{8}{3} \end{gathered}[/tex]Substitute the value of this constant in the general expression,
[tex]M=\frac{8}{3}\cdot\frac{d^2}{\sqrt[]{x}}[/tex]Thus, the required general formula to describe the relation is obtained as,
[tex]M=\frac{8}{3}\cdot\frac{d^2}{\sqrt[]{x}}[/tex]