Alright, let's work through 1686 times 864 step by step
1686 × 864 = 864 × (1686)
+ 864 × 6 which equals 5184
+ 864 × 80 that is equal 69120
+ 864 × 600 that is equal 518400
+ 864 × 1000 that results in 864000

= 1456704
