Edit: I corrected my wrong sign below ... Sorry for any confusion it caused.
I don't know what methods you've been taught . . .
Find the roots of the equation
Let , where and are real.
We have: .
Then: . +
Equate real and imaginary components: .
From (2), we have: .
Substitute into (1): .
We have two equations to solve:
Substitute into (2): .