There is some ambiguity here which could be removed by using parentheses. I'm going to assume that you actually meant:
x-3
h(x) = ---------------
(x^3-36x)
To determine the domain of this function, factor the denominator:
x^3 - 36x = x(x^2 - 36) = x(x-6)(x+6)
The given function h(x) is undefined when the denominator = 0, which happens at {-6, 0, 6}.
Thus, the domain is "the set of all real numbers not equal to -6, 0 or 6."
Symbolically, the domain is (-infinity, -6) ∪ (-6, 0) ∪ (0, 6) ∪ (6, +infinity).