My calculations:
##2a = (2a)^4 = 16a^4 = 16a##
##14a=0##
This allows for the characteristic to be 2 or 7. But, e.g., ##3^4 = 81 = 4 ~(mod~ 7)## rather than ##3##. So, the only answer is ##2##.
1. Is this derivation correct?
2. The hint in the textbook says to show that ##2a=0##. How to do that?