Alright, let's work through 3099 times 4200 step by step
3099 × 4200 = 4200 × (3099)
+ 4200 × 9 which equals 37800
+ 4200 × 90 which equals 378000
+ 4200 × 000 giving us 0
+ 4200 × 3000 which equals 12600000

= 13015800
