This equation involves no x variable, so I assume you want to solve this for D.
First of all, you can divide both sides by 0.785 to get
![0.785D^2 = 0.54 \iff (0.785D^2)/(0.785) = (0.54)/(0.785) \iff D^2 = (0.54)/(0.785) =](https://img.qammunity.org/2019/formulas/mathematics/middle-school/xd4tlb5ayozval1m6ny727nreoz7936qfl.png)
Since the right hand side is positive, the equation make sense (otherwise we would be asking a square, i.e. a positive number, to equal a negative number). So, we can extract the square root from both sides:
![D^2 = (0.54)/(0.785) \iff √(D^2) = \pm\sqrt{(0.54)/(0.785)} \iff D = \pm\sqrt{(0.54)/(0.785)}](https://img.qammunity.org/2019/formulas/mathematics/middle-school/e7vvo30nb0bvwmjdrowb8b1ygcqedp6vvv.png)