Program seems to work - though I have not rigourously testred the values. But I would be concerned about how "usable" it is for your grampa. That's a whole lot of boxes to type in and click to get the answer. You want to make the interface as intuitve as possible.
I know this is your first program, are you up to some more coding?
1] Don't ask, just provide the answer in both feet and inches. That saves one step AND gives him a choice of info.
2] As far as he is concerned, he does not care about a, b and c. All he cares about is base, height and hypotenuse, or possibly an angle. There is no need to ask him to enter a, b or c. That's a bit more coding, but it would be worth it.
3] Eventually (after you're more accomplished), your ideal interface will have no questions at all. It will display a triangle with some textboxes right in it. He will merely enter the values in the appropriate box by locating them visually rather than textually.