Hi,
A card player draws 4 cards from a standard pack of 52 cards
P(a card from each of the 4 suits) = 
the 39 , 36, 13 represent the number of cards left in other suits respectively as the next card is drawn
After 1st card .. there are 3 remaining suits to chose from, (13*3 = 39)
After 2nd card .. there are 2 remaining suits to chose from, (13*2 = 26)
After 3nd card .. there is 1 remaining suits to chose from, (13*1 = 13)