678. The number 43 is written in the septenary (base-7) numeral system. In which system is it written with the same digits but in reverse order?
$\triangle$ First, let's convert the number 43 to the decimal system:

$$
43_{7}=4 \cdot 7+3=28+3=31
$$

139

Now, let's determine the base $d$ of the system in which the equality $31=34_{d}$ holds. We will have:

$$
31=3 d+4, \quad 3 d=27, \quad d=9
$$

Answer: in base 9.