The way to figure this out is using factorial which is the !. So their are seven runners so it would be 7! (7*6*5*4*3*2*1). However it wants to only know first second and third so it changes things. This then makes it 7*6*5 because you can only have one racer finish in the 3 spots.
So your answer is 7*6*5=210
Hope this helped!