Answer:
a) December 3, 202x, merchandise sold to Sheridan Co., terms 3/10, n/30
Dr Accounts receivable 513,500
     Cr Sales revenue 513,500
Dr Cost of goods sold 318,300
     Cr Merchandise inventory 318,300
December 8, merchandise allowance 
Dr Sales returns and allowances 24,700
     Cr Accounts receivable 24,700
December 13, invoice collected from Sheridan Co.
Dr Cash 474,136
Dr Sales discounts 14,664 
     Cr Accounts receivable 488,800
b) January 2, invoice collected from Sheridan Co.
Dr Cash 488,800
     Cr Accounts receivable 488,800