Rearrange as
x^2 + 1x = -1
Take (1/2) of 1 = 1/2
Square it = 1/4 and add to both sides
x^2 + x + 1/4 -1 + 1/4 factor the left side and simplify the right
(x + 1/2)^2 = -3/4 take both roots
x + 1/2 = ± sqrt (-3 ) / 2
x + 1/2 = ± sqrt (3) i / 2
x = -1/2 ± sqrt (3) i / 2
x = [ -1 ± sqrt (3) i ] / 2