Is 2100x1500 the correct dimension for good quality developed digital camera pics if you want 5x7" pics?

Answer 1

Yes, 300 pixels per inch (ppi) works well with most home and commercial printers. So 2100x1500 is right on!

Printer driver software will factor it down if you give it a larger ppi. So you can save the image at higher resolution if you prefer.

But it cannot factor up if you give it too small ppi, so then print quality can suffer.

Moral of the story: For your important pictures, save at highest resolution you can handle, and only factor down for special purposes. Once you give away pixels, they are gone forever.