You can put this solution on YOUR website! We have a simple interest formula that says
I=Prt
Where I is the interest earned, P is the principal amount, r is the interest rate and t is the number of units of time that have passed (for example if the interest rate is annually, then t is the number of years that have passed).
So we have ((assuming your interest is annually, let me know if it is not))
So you have earned $18 in interest. So in total you have 300+18=$318 at the end of the year.