15a =6a - 90
Move all the numbers with variables to one side
15a - (6a) = 6a -(6a) -90
9a = -90
9a/9 = -90/9
a = -9
Hope this helps
So you would do,
18 gallons of water divided by 0.5 gallons of water
if you did that correctly you should have gotten... 36 jugs