Question 435715

3x-9/a^2+10a+21

a^2+10a+21 can not be zero
so find the roots of 
a^2+10a+21
factor 
(a+7)*(a+3)=0
and the roots are a=-7 and a=-3
check
49-70+21=0
9-30+21=0
so a cannot be -7 nor -3