Hi. I have been trying to solve this for weeks add can't figure it out.

I need to solve for 'b' in this equation:

(ie, I need to be able to reverse the equation so if given 'z' then I can find b. 'a' is just a constant).

$\displaystyle z = ab / sqrt (a^2 - b^2)$

Please if anyone can save me and show me how to do this. Thank you so much in advance.

(not sure if this image will show):