Alright, let's work through 3991 times 5389 step by step
3991 × 5389 = 5389 × (3991)
+ 5389 × 1 what gives us 5389
+ 5389 × 90 resulting in 485010
+ 5389 × 900 resulting in 4850100
+ 5389 × 3000 yielding 16167000

= 21507499
