Your substitution isn't quite right. First, yes, if we let

u = X+3

du = dx

But the next part is highly flawed. Though true that

x = u-3

We're interested in what x^2 is, because that's the numerator. You need to square both sides. Also, when trying to turn this into a product, you need to multiply the numerator by the inverse of the denominator, not add it.