Question

you are dealt one card from a 52-card deck. what is the possibility that you are not dealt a diamond

Answer 1

There are 4 suits, each with 13 cards (52/4)
The probability that you are NOT dealt a diamond is 39/52

(Could be any of the other 3 suits, 3x13=39 -> 39/52)

Answer 2

Probability is the total number of desired outcome over total outcomes possible. In a standard deck there are 13 cards in each suit. So you want to subtract 13 from 52 to get the number of desired outcomes. Then you put that over 52 and simplify.