Question

find any real numbers for which the rational expression is undefined x-3 over x+9

Answer 1

set the denominator equal to zero, and solve for \(x\) (in your head)
\[x+9=0\iff x=-9\]

Answer 2

an expression is undefined if the denominator is = zero

the denominator here is x + 9 so for x= -9 the denominator becomes zero and the expression is undefined