If we were to write this as a mathematical equation, it would look something like this:
3x + 7 = 4x + 5
In order to solve, we would need to isolate the variable. We can do this by subtracting 3x from both sides, after doing which, we are left with this:
7 = x + 5
Then, we subract 5 from both sides to isolate the variable
2 = x
That number you are looking for is 2.
Let's check out work by substituting 2 in for the unknown number in the original problem:
3(2) + 7 = 4(2) + 5
6 + 7 = 8 + 5
13 = 13
And we can see that the numbers do indeed check out.
Your answer is 2.
Hope that helped! =)