10.
When t=0, N=3000
3000=1800+Ae^0
A=1200
When t=3, N=3400
3400=1800+1200e^3k
1200e^3k=1600
3k=ln(16/12)
k=[ln(16/12)]/3
k=0.09589.....
a) N=1800+1200e^5k
=3738
b) 8000=1800+1200e^kt
6200=1200e^kt
e^kt=6200/1200
kt= ln(6200/1200)
t= [ln(6200/1200)]/k
t=17.13 yrs