At a rate of 13.5% the interest per $100 is $ 4.778 per month.
Therefore, for two years the payment will be
$4.778 × 24 = $114.672 per $ 100
Therefore, the interest for a principal $6250 for two years;
(6250/100) × 114.672
= $7167
The monthly payment would therefore be
7167/24
= $298.625
≈$298.63