The square root of -36 is the square root of -1 times the square root of +36.
The answer is 6i, where i is the square root of -1.
This can be checked by the standard formula for two complex numbers:
(a,b)x(c,d) = (ac-bd, ad+bc)
(0,6) x (0,6) = (0x0 - 6x6, 0x6 + 6x0) = (-36, 0) or just -36
