I'm so stuck yet probably so easy, what am I missing?...

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).

** **

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):*

http://24.82.3.191/MyPublic/publicImages/z.JPG

see, just like I said, easy....

...for a pro. Thank you so much! I get it now. I couldn't see the way but you make it seem easy.