Set the number of friend is x, the number of marbles is y.
everyone 11 marbles, left 6 marbles
So 11x + 6 = y (1)
everyone 14 marbles, red 12 marbles
So 14x - 12 = y (2)
according to (1), (2)
11x + 6 = y => x = 6
14x - 12 = y y = 72
So there are 6 friends, 72 marbles.