What you have is $\displaystyle T= I\sqrt{\frac{s}{d}}$. (And you want to solve for "d", not "D".)
You need to "undo" what has been done to d. If you were given values for I, s, and d, to find T you would
1) divide s by d.
2) take the square root.
3) multiply by I
To "undo" that, do the opposite of each step in the reverse order.
The reverse of "multiply by I" is to divide by I. Divide both sides by I to get
$\displaystyle \frac{T}{I}= \sqrt{\frac{s}{d}}$
The reverse of "take the square root" is to square. Square both sides to get
$\displaystyle \frac{T^2}{I^2}= \frac{s}{d}$.
The reverse of "divide by d" is to multiply by d. Multiply both sides by d to get
$\displaystyle \frac{dT^2}{I^2}= s$.
Since it is d we want to solve for, not s, divide both sides by $\displaystyle \frac{T^2}{I^2}$, which, of course, is the same as multiplying both sides by $\displaystyle \frac{I^2}{T^2}$ to get
$\displaystyle d= \frac{sI^2}{T^2}$