In the function (x2+1)/(6x2+13x-6), the domain is all values of x except for those which are zeroes of the denominator.
If 6x2+13x-6=0, x=(-13±√(169+144))/12=(-13±√313)/12=0.391 and -2.558 approx.
Therefore the domain is {(-∞,-2.558), (-2.558,0.391), (0.391,∞)}.