You get 5% for 10,000.
You get 8% anything over 10,000.
You sold 12,000, which means you had 2,000 more than 10,000.
Calculate the commission on each amount with the appropriate rate:
10,000 * 0.05 = 500
2,000 * 0.08 = 160
Total pay: 500 + 160 = $660