Варифметической прогрессии а2+а4=6,а6*а7=99. найти а1

A2 + a4 = 6
a6*a7 = 99
(a1 + d + a1 + 3d = 6
(a1 +5d)(a1 +6d) = 99

2a1 + 4d = 6
(a1 +5d)(a1 +6d) = 99

2a1 = 6 - 4d
a1 = 3 - 2a

(3-2d+5d)(3-2d+6d)=99
(3 =3d)(3 +4d) = 99
9 + 12d + 9d + 12d^2 - 99 = 0
12d^2  -21d - 90 = 0   / 3
4d^2  - 7d - 30 = 0
D = 49 +4*4*30 = 529
d1 = (7 -23)/8
d1 =  - 2
d2 = (7 + 23)/8
d2 = 3,75
a1 = 3 - 2*(-2) = 7
a2 = 3 - 2*(3,75) =  - 4,5
ответ: а1 = 7 и а2 = -4,5