this doesnt make sense to me

There are no solutions over the real numbers.

Over the complex numbers the two solutions are

x = +/- 6i

where i is the square root of -1

x^2 + 36  = 0  subtract 36 from both sides

x^2  = -36       take both roots

x  = ±√ - 36  = ±√ [ (-1)(36) ]  = ± i * 6  =  ± 6i

[ Remember that   √ (-1)  =  i  ]

Thanks CPhill and Rom