The 4th term is a(4) by the definition: a(4) = a(3) * 10 we need a(3)! a(3) = a(2) * 10 we need a(2)! a(2) = a(1) * 10 Ah we know that a(1) = -11! Now we can go back to a(4).
So now we know that a(2) = -110, that a(3) = -1100, And that a(4) = -11000 there's the answer