The thing to remember here is that the mass number, A, gives you the number of protons and the number of neutrons present inside a given atom's nucleus.
color(blue)(|bar(ul(color(white)(a/a)A = Z + "no. of neutrons" color(white)(a/a)|)))
Here
Z - the atomic number of the atom
In your case, the "X"^(3-) anion is said to have a mass number equal to 14. This tells you that this particular ion has a total of 14 nucleons, which as you know is a term used to describe a proton or a neutron, inside its nucleus.
You can thus say, using the above equation, that your anion has
14 = Z + "no. of electrons"
Now, you know that this anion has 10 electrons. The charge of an ion can be calculated by subtracting the number of electrons that surround the nucleus from the number of protons that are located inside the nucleus
"charge" = Z - "no. of e"^(-)
In your case, a 3- charge tells you that this anion contains 3 additional electrons than a neutral atom of "X" would contain. This means that you have
-3 = Z - 10 implies Z = 7
You now know that an atom of "X" has 7 protons inside its nucleus. As a result, the number of protons located inside the nucleus of "X" will be
color(green)(|bar(ul(color(white)(a/a)color(black)("no. of neutrons" = 14 - 7 = 7)color(white)(a/a)|)))
Moreover, you can say that you're dealing with the nitrogen-14 isotope, ""^14"N", the most abundant of the two stable isotopes of nitrogen.
The "X"^(3-) anion is the nitride anion, "N"^(3-).
