х² - dх + 15 = 0
х₁ = 5
5² - 5d + 15 = 0
25 + 15 - 5d = 0
40 - 5d = 0
5d = 40 - 0
d = 40 : 5
d = 8
- - - - - - - - - -
х² - 8х + 15 = 0
D = b² - 4ac = (-8)² - 4 · 1 · 15 = 64 - 60 = 4
√D = √4 = 2
х₁ = (8+2)/(2·1) = 10/2 = 5
х₂ = (8-2)/(2·1) = 6/2 = 3
Вiдповiдь: d = 8; х₂ = 3.