x2 x =0

-4x2 +17x +21 = 0

ax2 bx c = 0

a = -4

b = 17

c = 21

Δ = b2-4ac

Δ = 17² - (4 * -4 * 21)

Δ = 289 - (4*-84)

Δ = 289 - (-336)

Δ = 625      Δ > 0

-4x2+17x+21=0

x = -b ∓ √Δ 2a

x' = -17 - √625 2*-4 = -17 - 25.0 -8 = -42.0 -8 = 5.25

x" = -17 + √625 2*-4 = -17 + 25.0 -8 = 8.0 -8 = -1.0

x' = 84.0

x" = -16.0

S = {x ∈ R / x' = 84.0, x" = -16.0 }