Step: 1
A number z is multiplied by 3 = 3 × z = 3z
Step: 2
3z is decreased by 15 = 3z - 15
Step: 3
3z - 15 = 12
[Original equation.]
Step: 4
3z = 12 + 15
[Add 15 to each side.]
Step: 5
Step: 6
z = 273 = 9
[Divide each side by 3 and simplify.]
Step: 7
So, the number z is 9.