Complete the following series: N7V, K9T, H12R, __?__, B21N

The missing term is E16P.

The first letters N, K, H fall by three each, so the next is E and the last is B.

The numbers 7, 9, 12 rise by two, then three, then four, giving 16, and then five to reach 21.

The last letters V, T, R fall by two each, so the next is P and the final is N.

Combining a first letter E, the number 16 and a last letter P gives E16P.