12 r - 18 = 13 r + 18

First you want to isolate the variable, do this by adding 18 to both sides. You're left with 12r = 13r + 36, then subtract 13r to get the variables alone. You're then left with - 1r = 36, multiply by - 1 to remove the negative. The final result would be (1) r = 36.

12r-18=13r+18

-12r. - 12r

1r-18=18

+18 + 18

1r = 36

1r/1=36/1

r=36