Realize that
f(1) = 14,000,
f(2) = f(1) * 0.9;
f(3) = f(2) * 0.9;
f(4) = f(3) * 0.9 ,...
So, the answer is:
f(1) = 14,000
f(n) = f(n-1) * 14,000 , for n ≥ 2
And after 3 years =>
f(3) = f(2)* 0.9 = f(1) * 0.9 * 0.9 = 14,000 * 0.9 * 0.9 = 11,340 birds will remain