I don't think that there is a solution to this. Consider the following two equations:

and

We should be able to solve this system. So from the second equation we get

Mind you this is in mod 729. So we need the multiplicative inverse of 4 in mod 729. This is 547. Thus

Put this value for a in the first equation:

etc, etc. I get a = 65 and b = 158. But

which is not 5 as your equations specifiy.

-Dan