(х - 3)(6х + 5) = (х - 3)(2х - 3),
6х² + 5х - 18х - 15 = 2х² - 3х - 6х + 9,
6х² - 13х - 15 = 2х² - 9х + 9,
6х² - 13х - 15 - 2х² + 9х - 9 = 0,
4х² - 4х - 24 = 0,
х² - х - 6 = 0.
D = (-1)² - 4 · 1 · (-6) = 1 + 24 = 25; √25 = 5
x₁ = (1 + 5))(2 · 1) = 6/2 = 3
x₂ = (1 - 5)/(2 · 1) = -4/2 = -2