Hello from MrBillDoesMath!
Answer: b = -2
Discussion:
Let's analyze this sequence.
The first number = -5 a(0)
The second number = 10 = -5 (-2) a(1) = a(0) *-2
the third number = -20 = 10 * (-2) a(2) = a(1) * -2
The fourth number = -20 * (-2) a(3) - a(2) * -2
In other words, the recursive (not reclusive) formula is
a(0) = -5 (this need to be stated! Not stated in the problem)
a(n) = a(n-1) * (-2)
This implies that "b" = -2.
Regards, MrB