Answer:
r=![-\frac{S}{L} +1](https://tex.z-dn.net/?f=-%5Cfrac%7BS%7D%7BL%7D%20%2B1)
Step-by-step explanation:
You would want to isolate the r by itself by doing all the opposite operations.
First, you can divide the L to get
1-r =![\frac{S}{L}](https://tex.z-dn.net/?f=%5Cfrac%7BS%7D%7BL%7D)
Next, move the 1 to the other side to get
-r=![\frac{S}{L} -1](https://tex.z-dn.net/?f=%5Cfrac%7BS%7D%7BL%7D%20-1)
Then, we would not want to keep the negative so, move it to the other side to get
r= ![-\frac{S}{L} +1](https://tex.z-dn.net/?f=-%5Cfrac%7BS%7D%7BL%7D%20%2B1)
(Note the 1 changed to a positive because we move the negative to the other side so, negative or negative 1 will be positive.