For this function denominator should not be equal "0".
We need to find when the denominator is equal zero.
(x+3)(x-1)=0
x+3=0, x=-3,
x-1=0, x=1
The domain is going to be all real number but without points -3, and 1.
I can be written as
(-∞,-3)∪(-3,1)∪(1,+∞)