Alright, let's work through 6399 times 6590 step by step
6399 × 6590 = 6590 × (6399)
+ 6590 × 9 producing 59310
+ 6590 × 90 giving us 593100
+ 6590 × 300 giving us 1977000
+ 6590 × 6000 giving us 39540000

= 42169410
