Alright, let's work through 6440 times 1688 step by step
6440 × 1688 = 1688 × (6440)
+ 1688 × 0 which equals 0
+ 1688 × 40 that equals 67520
+ 1688 × 400 that results in 675200
+ 1688 × 6000 that results in 10128000

= 10870720
