If you want to solve for x you have to get x alone on one side of the equals sign. In order to do that you have to get rid of the stuff on the same side by doing the opposite operation so that it cancels. What you do on one side you do on. the other side to keep the equation true.
y= abx
a and b are being multiplied by x so divide by ab to cancel them out (ab/ab is 1).
y= abx/ab (ab cancels)
now divide the other side by ab to keep the equation true.
y/ab= x