If it's a linear function under the square root, just substitute u for that entire linear function.
Wrong, I'm afraid:
then
then
then
is correct.
One good reason for using rather than is because then it's obvious what the sign is. is always positive.
You should be able to find all this stuff in a basic primer on calculus: there is invariably a list of "useful" substitutions if the book you're using is worth a damn. If not, then find them on line.