The phrase a number means an arbitrary variable. We can pick any letter a-z except for i and e.
Let's choose x.
3 times a number:
3x
The sum of three times a number and twelve means we add 12 to 3x:
3x + 12
Let's choose x.
3 times a number:
3x
The sum of three times a number and twelve means we add 12 to 3x:
3x + 12