5x2+20x+20=0

Δ = b2-4ac

Δ = 20² - (4 * 5 * 20)

Δ = 400 - (4*100)

Δ = 400 - (400)

Δ = 0      Δ = 0

x = -b 2*a

x = -20 2*5

x = -2.0

S = {x ∈ R / x = -2.0}