Alright, let's work through 2561 times 6638 step by step
2561 × 6638 = 6638 × (2561)
+ 6638 × 1 which equals 6638
+ 6638 × 60 that is equal 398280
+ 6638 × 500 what gives us 3319000
+ 6638 × 2000 which equals 13276000

= 16999918
