Write a general formula to describe the variation.
$M$ varies directly with the square of $d$ and inversely with the square root of $x ; M=12$ when $d=3$ and $x=25$

Step 1 ：Given that $M$ varies directly with the square of $d$ and inversely with the square root of $x$, we can write the relationship as $M = k \cdot \frac{d^2}{\sqrt{x}}$, where $k$ is the constant of proportionality.

Step 2 ：Substitute the given values of $M$, $d$, and $x$ into the equation to find the value of $k$.

Step 3 ：Given: $M = 12$, $d = 3$, $x = 25$

Step 4 ：Solving for $k$, we get $k = 6.67$

Step 5 ：Substitute the value of $k$ back into the equation to get the general formula that describes the variation of $M$ with respect to $d$ and $x$.

Step 6 ：The general formula that describes the variation is $M = \boxed{6.67} \cdot \frac{d^2}{\sqrt{x}}$