what would be the amount of a check issued for an 08/11 invoice for 648.50 paid on 09/13 with the terms: 3/10, 2/15, N30-5%APR?

The amount of a check issued for an 08/11 invoice, would depend on when the invoice is paid.

If it's paid within 13 days, there's a 3% discount. which will be calculated as

648.50 x 3% = $19.455 discount

If it's paid within 11 - 15 days, there's a 2% discount.

648.50 x 2% = $12.97 discount

If it's paid within 16 - 30 days, there is no discount

If it's paid after 30 days, 5% APR is added to the bill