William gave 4 football cards to each of his 6 friends. suppose he had 54 cards left. write and solve an equation to find how many cards each friend initially had.

For this case, the first thing we must do is define a variable.
 We have then:
 x: number of initial cards.
 We write now the equation that represents the problem:
 x - (4) * (6) = 54
 From here, we clear the value of x:
 x = (4) * (6) + 54
 Rewriting:
 x = 24 + 54
 x = 78
 
 The number of cards that william has initially is 78. Lack of information to know how many cards his friends have.