Initially, you invest $9000 at 4.3% p.a. (which is 4.3/12= 0.358% per month) so compounded monthly (24 times in two years) it willl be worth $9000(1.00358)^24 at the end.
Now $100 is added after the first month so it is compounded monthly for 23 months and will, at the end be worth $100(1.00358)^23
Now $100 is added after the second month so it is compounded monthly for 22 months and will, at the end be worth $100(1.00358)^22.
Now $100 is added after the third month so it is compounded monthly for 21 months and will, at the end be worth $100(1.00358)^21.
Continue like that until $100 is compounded monthly for 1 month and will, at the end be worth $100(1.00358).
At the end of two years, the investment will be worth
\(\displaystyle 9000(1.00358)^{24}+ 100\left(\sum_{n=1}^{23}(1.00358)^n\right)\).
