If you are looking for the total interest to be paid you would use the formula: loan amount X interest = total loan amount.
convert your 4.5% into a decimal by dividing it by 100. Your decimal will be 0.045
$2,500 X 0.045 = $112.50 total interest amount
If you want how much interest you will pay each month use this formula: interest amount / number of months.
$112.50 / 18= $6.25 per month