-18x2 +13x +21 = 0
ax2 bx c = 0
a = -18
b = 13
c = 21
Δ = b2-4ac
Δ = 13² - (4 * -18 * 21)
Δ = 169 - (4*-378)
Δ = 169 - (-1512)
Δ = 1681 Δ > 0
-18x2+13x+21=0
x = -b ∓ √Δ 2a
x' = -13 - √1681 2*-18 = -13 - 41.0 -36 = -54.0 -36 = 1.5
x" = -13 + √1681 2*-18 = -13 + 41.0 -36 = 28.0 -36 = -0.777777777778
x' = 486.0
x" = -252.0
S = {x ∈ R / x' = 486.0, x" = -252.0 }