Hi there. I'm new here :-)

I've been going through a fair few substitution questions, but I have been stumped on this one.

I have to solve the following by substitution:

Now normally I find these questions relatively easy, where all the values under the square root are substituted for u.

This question does not seem to work the same way since or

Clearly this will not replace the term outside the square root.

Any ideas? :-)