You've done all the hard stuff.
3x+7 is given in the question as the value of the definite integral.
You have calculated the same integral as having a numeric value of 3x-3a.
So 3x+7 = 3x-3a. solve for a
If you're still confused you are probably over-thinking the problem. You have two expressions for the same definite integral, so they must be equal.