At death means x= 0, so on the graph you can see that (0, 500), so 500 carbon atoms are present at death.
Plug 11,240 in for t in the equation, and you are left with N(t) = 500(1/2)^2 = 125 carbon atoms.
See the graph point (25,000, 24.298)? That means that after 25,000 years 24.298 carbon atoms will be present. So the answer is 25,000 years.