x^2+4=0

x² + 4  =  0

x²  =  -4

If you have only real numbers, there will be no solution, but if you have complex numbers:

x  =  ±√-4

x  =  ±√4·√-1       where  √4 = 2  and  √-1  can be replaced with the symbol  i

x  =  ±2i