You need to have variables one one side and constants on the other. ax - ax + by = c - ax
by = c - ax
by/b = (c - ax)/b
y = (c - ax)/b
B is your answer! All you do for this equation is go oh look m=2/7 well this means y=2/7x and you have a -12 so than you just plug it in. To get y=2/7x-12
Moves the decimal over two places to the right and that will be your percentage.