The formula for average atomic mass is :
mass of isotope A * % of isotope A + mass of isotope B * % of isotope B + ....
Now,
Here,
Average atomic mass of nitrogen = (14.003 * 99.63%) + (15.000 * 0.37%)
= (14.003 * 0.9963) + ( 15.000 * 0.0037)
= 13.951 + 0.056
= 14.007 a.m.u.