Let the number be X.
2X + 5 = 4X - 7
Therefore 4X - 7 = 2X + 5
Now all of the Xs need to be on one side:
4X - 7 - 2X = 5
2X - 7 = 5
Now put the single numbers all on the other side:
2X = 5 + 7
2X = 12
And finally divide by the number in front of the X:
2X / 2 = 12 / 2
X = 6
So the original number is 6.