a(7) = -8√2
a(n) = a(1)•q^(n-1)
a(4) = a(1)•q^3 = -4
a(6) = a(1)•q^5 = -8
a(6) : a(4) = q^2 = 2 => q = √2
a(7) = a(6)*q = -8√2
