36 is 12 percent of what number?

1. We assume, that the number 12 is 100% - because it's the output value of the task. 
2. We assume, that x is the value we are looking for. 
3. If 100% equals 12, so we can write it down as 100%=12. 
4. We know, that x% equals 36 of the output value, so we can write it down as x%=36. 
5. Now we have two simple equations:
1) 100%=12
2) x%=36
where left sides of both of them have the same units, and both right sides have the same units, so we can do something like that:
100%/x%=12/36
6. Now we just have to solve the simple equation, and we will get the solution we are looking for.

7. Solution for 36 is what percent of 12

100%/x%=12/36
(100/x)*x=(12/36)*x       - we multiply both sides of the equation by x
100=0.333333333333*x       - we divide both sides of the equation by (0.333333333333) to get x
100/0.333333333333=x 
300=x 
x=300

now we have: 
36 is 300% of 12

Y=-2x-6 is the answer