Alright, let's work through 3639 times 721 step by step
3639 × 721 = 721 × (3639)
+ 721 × 9 what gives us 6489
+ 721 × 30 yielding 21630
+ 721 × 600 yielding 432600
+ 721 × 3000 which equals 2163000

= 2623719
