Alright, let's work through 3981 times 634 step by step
3981 × 634 = 634 × (3981)
+ 634 × 1 that is equal 634
+ 634 × 80 resulting in 50720
+ 634 × 900 giving us 570600
+ 634 × 3000 what gives us 1902000

= 2523954
