solve variables in terms of variables

Help would be appreciated.

1.)

Solve this formula for the variable $\displaystyle \mu$:

z = $\displaystyle \frac{x-\mu}{\mu}$

2.)

Solve this formula for the variable R:

$\displaystyle \frac{E}{e}$ = $\displaystyle \frac{R+r}{R}$

3.)

The intensity of light as measured from a light bulb varies inversely as the square of the distance "d" from the bulb. Suppose that I is 90W/m^2 when the distance is 5 m. What would the light intensity be 10m from the bulb?

4.) Write a radical expression with an index of 3 and a radicand of (x - 5)^9. Simplify it as much as possible.

Help with an explanation of the steps would be very much appreciated. I'm completely lost and didn't get a chance to meet with the instructor after class because of work.