help me please.

$\frac{f}{r^2}+a=n+k$

Subtract a from each side.

$\frac{f}{r^2}=n+k-a$

Multiply each side by r^2.

$f=(n+k-a)\times r^2$

Divide each side by (n+k-a).

$\frac{f}{n+k-a}=r^2$

And then take the square root of each side, and you get an equation equal to r.

I'll leave this last step for you to do.