In the given problem there are several vital informations to take note off for completing the required answer to the question.
It has been assumed that the unknown number is = p
Now we can get to the desired equation for solving and finding the value of p.
(p + 7)/12 = 3
p + 7 = 3 * 12
p + 7 = 36
p = 36 - 7
= 29
So the value of the unknown number p is 29. I hope that the method i have used to solve this problem is clear to you and you can solve such type of problems in future without anyone's help.