If you would like to transform the equation 6*x + 2*y = 15 into the equation 0 = 15 - 6*x - 2*y, you have to do two following steps or you can do this at the same time (and you have to do just one step).
First, you have to subtract 6*x from both sides of the equation:
6*x + 2*y = 15 /-6*x
6*x + 2*y - 6*x = 15 - 6*x
2*y = 15 - 6*x
Second, you have to subtract 2*y from both sides of the equation:
2*y = 15 - 6*x /-2*y
2*y - 2*y = 15 - 6*x - 2*y
The result:
0 = 15 - 6*x - 2*y
