not sure if
[tex]D=\frac{1}{2fk}[/tex]
or
[tex]D=\frac{1}{2}fk[/tex]
first one
[tex]D=\frac{1}{2fk}[/tex]
times both sides by k and divide both sides by D
[tex]k=\frac{1}{2fD}[/tex]
2nd one
[tex]D=\frac{1}{2}fk[/tex]
times both sides by [tex] \frac{2}{f} [/tex]
[tex] \frac{2D}{f}=k [/tex]
[tex] k= \frac{2D}{f} [/tex]
