Alright, let's work through 9991 times 9990 step by step
9991 × 9990 = 9990 × (9991)
+ 9990 × 1 resulting in 9990
+ 9990 × 90 yielding 899100
+ 9990 × 900 what gives us 8991000
+ 9990 × 9000 that is equal 89910000

= 99810090
