The phrase a number means an arbitrary variable. We can pick any letter a-z except for i and e.
Let's choose x.
Twice a number means we multiply x by 2:
2x
Decreased by six means we subtract 6 from 2x:
2x - 6
Let's choose x.
Twice a number means we multiply x by 2:
2x
Decreased by six means we subtract 6 from 2x:
2x - 6