Alright, let's work through 8639 times 931 step by step
8639 × 931 = 931 × (8639)
+ 931 × 9 producing 8379
+ 931 × 30 what gives us 27930
+ 931 × 600 what gives us 558600
+ 931 × 8000 giving us 7448000

= 8042909
