Puzzle ZFFC 

Workings

Hint
Look backwards.

Answer
nogatco.

Reasoning
These are geometric shapes, the number of sides increases, and the words are reversed.
3 elgnairt = triangle
4 erauqs   = square
5 nogatnep = pentagon
6 nogaxeh  = hexagon
7 nogatpeh = heptagon
8 nogatco  = octagon