Answer
$$-.8,-3,3$$
Work Step by Step
The fraction is undefined for any number that makes the denominator of the individual fractions or the denominator of the overall fraction zero. Thus, we find that the given expression will be undefined for:
$$ 5x+4=0 \\ x=-.8$$
AND:
$$ \frac{3}{7}=\frac{x^2}{21}\\ x=3,\:x=-3$$