S=l (1 - r), for r
this is all it says, you are solving for r, the instructions just say solve for r

OK. Then let's solve for ' r '.  That means you have to come up with an equation that says     r =    everything else.

Step #1: 
Write the equation you're given:   S = L (1 - r)

Let's divide each side by ' L ':      S/L = 1 - r

Subtract 1 from each side :         S/L - 1 = -r

Multiply each side by -1 :            1 - S/L = r

and there you have it.