Alright, let's work through 6643 times 3872 step by step
6643 × 3872 = 3872 × (6643)
+ 3872 × 3 which equals 11616
+ 3872 × 40 which equals 154880
+ 3872 × 600 that results in 2323200
+ 3872 × 6000 what gives us 23232000

= 25721696
