i assume we are dealing with a 52-card deck that has no jokers. There are 52 cards in all, half of them are black, and 4 of them are 7's

so there are 26 possible black cards we can draw from the deck of 52, and 4 7's we can draw out of the deck of 52

when we want to find the probability of one event OR another event happening, we add their respective probabilities.

thus, the probability of drawing a black card or a 7 is: ...i leave simplifying that to you

EDIT: i forgot to account for repeats. that is, what if the black card is a 7? see Plato's solution on how to deal with this