What is the answer to y=9x

There is no answer.  It is just the equation of a line.

x = y/9

Cause you need always the contrary operation. If you want to cut 9x you must remember that 9x means 9*x and the opposite of it is /. 

So here is the detailed way:

                   y=9x   | make clear all operations

<=>            y=9*x  | change (Commutative property)

<=>            y=x*9  | put the 9 on the other side (both sides /9)

<=>            y/9 = (x*9)/9  | summarize

<=>            y/9 = x