Answer :
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
[tex] 0.785D^2 = 0.54 \iff \dfrac{0.785D^2}{0.785} = \dfrac{0.54}{0.785} \iff D^2 = \dfrac{0.54}{0.785} = [/tex]
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:
[tex] D^2 = \dfrac{0.54}{0.785} \iff \sqrt{D^2} = \pm\sqrt{\dfrac{0.54}{0.785}} \iff D = \pm\sqrt{\dfrac{0.54}{0.785}}[/tex]
Ok so move the 0.785 to the right side D^2=0.54-0.785 D^2=68789808 Square root both sides D= +- 0.82939621