Answer
$^{15}_7\textrm{N}$
Work Step by Step
The top number is the mass number, equal to $\textrm{number of neutrons + protons}$. In the case of the radioactive nitrogen isotope, there are $8$ neutrons instead of $7$. The number of protons doesn't change, since we're dealing with nitrogen and nitrogen is defined as being the element with $7$ protons. Thus, the mass number is $7+8=15$.
The bottom number is the atomic number, which is equal to the number of protons. We've already determined this to be $7$.
The symbol for the element nitrogen is $\textrm{N}$.
