Prove that the equation x² + (a-2d)x + (ab - ad - b²) = 0 has real roots for all values of a, b and d.

Here's what I've done so far (correct me if I've done something wrong btw) ...

a= 1 b = a-2d c= ab - ad - b²

b² - 4ac ≥ 0

(a-2d)² - 4(1)(ab-ad-b²) ≥ 0

so if I'm thinking right, I have to prove the left side is positive? I've been trying different things but I'm stuck ... can somebody help me please?