Точно не знаю, но наверное, как-то так
X^2 - 14x + 13 = 0
x1 + x2 = 14
x1 * x2 = 13
--------------------------
x^2 + 12x + 35 = 0
x1 + x2 = - 12
x1 * x2 = 35
----------------------------
7x^2 - 2x - 14 = 0
x1 + x2 = 2 / 7
x1 * x2 = ( - 14 ) / 7 = - 2
g=a2/a1=-6/3=-2
a3=a1*g^2=3*4=12. a4=a1*g^3=3*(-8)=-24. a5=a1*g^4=3*16=48
5х-7х-7=9
-2х=9+7
-2х=16
х= - 8