Alright, let's work through 3991 times 4231 step by step
3991 × 4231 = 4231 × (3991)
+ 4231 × 1 that results in 4231
+ 4231 × 90 producing 380790
+ 4231 × 900 yielding 3807900
+ 4231 × 3000 that is equal 12693000

= 16885921
