-6x2 +15x +21 = 0
ax2 bx c = 0
a = -6
b = 15
c = 21
Δ = b2-4ac
Δ = 15² - (4 * -6 * 21)
Δ = 225 - (4*-126)
Δ = 225 - (-504)
Δ = 729 Δ > 0
-6x2+15x+21=0
x = -b ∓ √Δ 2a
x' = -15 - √729 2*-6 = -15 - 27.0 -12 = -42.0 -12 = 3.5
x" = -15 + √729 2*-6 = -15 + 27.0 -12 = 12.0 -12 = -1.0
x' = 126.0
x" = -36.0
S = {x ∈ R / x' = 126.0, x" = -36.0 }