what do you mean by "solve"?
f(x) = √(2x+1)
doesn't involve a difference quotient.
If you want to write down the difference quotient, then that would be
= (√(2(x+h)+1) - √(2x+1))/h
= (√(2x+1+2h) - √(2x+1))/h
Not sure where you want to go with that . . .