Percentage profit on investment

I did maths A-level but this is confusing me more than it perhaps should:

On an 18month investment. The first 12 months had a capital input of 7200, the last six months had a further 10200 invested brining the total capital input up to 17400. At the end of the 18 months the profit was exactly 3000.

Assuming the annual rate of interest is constant regardless of the amount invested, what is the average annual rate of interest as expressed in a % form. Please show your working.

Thanks