Дробь: x^2+4x/x+3 = 2x+3/3;

(х² + 4х) / (х+3) = (2х + 3) / 3,

3*(х² + 4х) / (х+3)*3 = (2х + 3)(х + 3) / (х + 3)*3,

(3х² + 12х) / (х+3)*3 = (2х² + 6х + 3х + 9) / (х + 3)*3,

(3х² + 12х - 2х² - 6х - 3х - 9) / (х + 3)*3 = 0,

(х² + 3х - 9) / (х + 3)*3 = 0,