There is only one 6 of Clubs in a standard pack of 52 playing cards, so the probability of selecting it is 1/52, the same as selecting any other specific card.
There are 4 suits so there is a probability of 13/52=1/4 of selecting any Club; and a probability of 1/13 in selecting a 6 of any suit. The probability of selecting a Club and a 6 is 1/4*1/13=1/52.
The probability of selecting a diamond or a 3 is 1/4+1/13=13/52+4/52=17/52.