6_ _ _ :9*8*7=504
_6_ _ : 8*8*7=448 ( we can't take 0 at first place therefore the first place will have only 8 choices and for the next place we will have 8 choices because 0 has been included and for the next 7 choices )
_ _6 _: 8*8*7=448
_ _ _6: 8*8*7=448
Total=504+448+448+448=1848