you'll make calculations (and life) much easier for yourself if you use the "store" capability of the graphing calculator.
to store a value ...
1. type out the value
2. hit the [STO] button
3. hit the [ALPHA] button and select the letter you want to store the value into.
4. press [ENTER] to store the value
for example, to store the universal gravitational constant, your screen will display ...
6.673E-11 -> G
hit [ENTER] and it's done.
store the mass of the planet in M ...
6.392E23 -> M
add the planet radius and the height and store in R ...
(did you remember to change km to m for h?)
3.395E6 + 3.41E5 -> R
now all you have to do is type in and calculate ...
you should get about 4778 m/s which you can change to km/s if that's the way you need to express it.