Question 323019
To make something 'undefined', the easy way to think about it is "what numbers will put a zero in the denominator", as anything divided by zero is undefined.

So what number(s) multiplied by -9 = 0?

You know immediately the answer is 0, but for more complex denominators or if you have to show work...

Set the denominator equal to zero
{{{-9y=0}}}
{{{(-9y)/-9=0/-9}}}
{{{y=0}}}