Alright, let's work through 8139 times 644 step by step
8139 × 644 = 644 × (8139)
+ 644 × 9 yielding 5796
+ 644 × 30 that is equal 19320
+ 644 × 100 yielding 64400
+ 644 × 8000 resulting in 5152000

= 5241516
