manipulating the centered difference approximation

[sys5284]

The equation is attached in the image file. I tried several times to work it out but I can't figure this out for the life of me. They just want me to show that the first equation equals the second one. Any atmospheric/meteorology majors out there? Any help in general would be appreciated.

[LawrenceC]

Treat it as a*x**2 + b*x + c = 0 and solve for x. The radical sign should extend over the division by R.