Alright, let's work through 3990 times 724 step by step
3990 × 724 = 724 × (3990)
+ 724 × 0 producing 0
+ 724 × 90 what gives us 65160
+ 724 × 900 what gives us 651600
+ 724 × 3000 yielding 2172000

= 2888760
