Alright, let's work through 3990 times 8391 step by step
3990 × 8391 = 8391 × (3990)
+ 8391 × 0 that is equal 0
+ 8391 × 90 giving us 755190
+ 8391 × 900 what gives us 7551900
+ 8391 × 3000 producing 25173000

= 33480090
