-x2+8x-16=0

Δ = b2-4ac

Δ = 8² - (4 * -1 * -16)

Δ = 64 - (4*16)

Δ = 64 - (64)

Δ = 0      Δ = 0

x = -b 2*a

x = -8 2*-1

x = 4.0

S = {x ∈ R / x = 4.0}