# How do you find all the complex roots of x^2 + 8x + 21=0?

x^2+8x+16+5=0=>(x+4)^2=-5=>(x+4)^2=5i^2=> x_1=-4+sqrt5*i , x_2=-4-sqrt5*i
where $i$ is the imaginary unit with ${i}^{2} = - 1$