Okay, I know this should be easy, but I'm having right bother with it the question goes:
All I know is I need to find the right substitution. Can anyone help?
Since you mention hyperbolic functions, , and so while . If we subtract the second from the first, all terms involving cancel leaving or . In order to use that with you want to make those look alike. One way to do that is factor out 9: . Now the substitution cosh t= (2/3)x gets rid of the square root.