how many electrons does nitrogen gain in order to achieve a noble-gas electron configuration? is it 4?

Nope, it's 3. Looking across period 2, you can see that Nitrogen has 5 valence electrons. It wants 8. 8-5=3.

Thanxs. How many significant figures are in the measurement 811.40 grams?

Five. Here is a good site for s.f.
http://science.widener.edu/svb/tutorial/sigfigures.html

how many significant figures are in 811.40

convert the rate 60m/s to units of km/hr?