Fine the average rate of change by the difference in N over the difference in T:
3000 - 500 / 4-0 = 2500 / 4 = 625
This means for every 1 T, N = 625.
Now multiply 625 by 8:
625 x 8 = 5000
When T = 8 N = 5,000
The decay rate can be found by dividing -ln(2) by the half life.
Decay rate = -ln(2) / 5715 = -0.00012