Solve 3^-x = 243

3^-x = 243

This says that

1/3^x = 243   which implies that

3^x = 1/243   but 243 = 3^5   ... so.... 1/243 = 1/3^5  = 3^-5  ...so...

3^x = 3^-5   and it's obvious that x = -5