Hi.. I'm following along in a book for my Discrete Mathematics class and I'm having trouble finding out what's going on between two steps algebraically.

21-1(77-3 x 21)

= 4 x 21 - 1 x 77

Exactly what is happening between these two steps? I can't see where the 4 comes from.

(Note: The xs in this problem stand for multiplication, not a variable x)

Thank you.