x^2 - 5x + 6 = 0;

D = b^2 - 4ac = (-5)^2 - 4 * 1 * 6 = 25 - 24 = 1;

x1 = (-b + √D)/2a = (5 + √1)/2 * 1 = (5 + 1)/2 = 6/2 = 3;

x2 = (-b - √D)/2a = (5 - √1)/2 * 1 = (5 - 1)/2 = 4/2 = 2.