Look at the periodic table to find the charge on atoms.
Magnesium is +2 and Nitrogen is -3. Since there are two nitrogen charge 2*-3 = -6 there needs to be 3 Mg then (3*2+ = 6+) to pair with the two nitrogen.
3 Mg(+2) + 2 N(-3) = Mg3N2
Answer: Low melting points
Explanation:
To find the third side you would use Pythagorean theorem which is a²+b²=c².
A and B being the 2 legs and C being the hypotenuse.
Example: ( sorry for the really odd not really a triangle, triangle...)
| \
6| \ ? 6²+9²=C²
|_______ 36+81=C²
9 117=C²
√117=10.8 (rounded)
if you need to get the leg you just fill in the numbers.