there is no square root with real number for a negative number, but you can use imaginary numbers such as i which means the square root of -1
so you can factor the -1 out and get 256
you factor 256 into 2*2*2*2*2*2*2*2 or 2^8
so if you group them into (2^2)(2^2)(2^2)(2^2) then square root all of them you get 2^4 or 16
but there is the immaginary part so the answer is 16i