since A1 is just half the total yearly payment (annuity) without any interest for first 4 years.
Hence for 4 years total money to be repaid=2000$ , A1 is for 1 year so A1=2000/4=500$.
interest for these 4 years = 4000$ * (1+0.09)^4 - 4000 =1646.32$
since 2000$ has been repaid, total amount to repay now = 2000$+1646.32$=3646.32$
This has to be paid in the form of uniform annuity, here comes the present value concept.
The present value of annuity has to be determined for the annuities you are paying in 5th,6,7,8th years, that present value(pv) is equal to 3646.32$.
pv= A2(1/ (1+0.09) + 1/ (1+0.09)^2+1/ (1+0.09)^3+1/ (1+0.09)^4)=3646.32$.
A2* 3.23971 = 3646.32$
so
A2 = 1125.50$
and we got A1=500$