How does a positive 6x(2nd power)y-36x(2nd power)y end up equaling positive 42x(2nd pwer)y?

$$\\6x^2-36x^2\\
=(6-36)x^2\\
=-30x^2\\$$

 

If this is now equal as below

 

$$-30x^2=42x^2y$$

divide both sides by 6x^2

$$\\-5=7y\\
y=\frac{-5}{7}$$

 

So the only time they will be equal is when y=-5/7