Thread: Integration with a square root at the bottom

1. Integration with a square root at the bottom

So I was reviewing my examples, and one of them looked like this:

and that u = 4x + 1, therefore du = 4dx. But how did the first step above turned into the next step? I can't seem to see how the next part came to be.

2. Actually, the substitution given is $u^2=4x+1.$