Задача с прогрессиями

$\left \{ {{ a_{2}+ a_{4}+ a_{6} =18} \atop { a_{2}* a_{4}* a_{6} =-168}} \right.$
a₂ = a₁+d
a₄ = a₁+3d
a₆ = a₁+5d
a₂+a₄+a₆ = 3a₁+9d = 18
a₁ + 3d = 6
a₁ = 6-3d

a₂*a₄*a₆ = (a₁+d)(a₁+3d)(a₁+5d) = (6-2d)*6(6+2d) = -168
(6-2d)(6+2d) = -28
36-4d² = -28
4d² = 64
d² = 16
d=4 или d=-4
a₁ = 6-12=-6 a₁= 6+12 = 18