Answer
For $z = a + bi$,
if $(a, b)$ is in the Julia set,
the conjugate of $z$,
$\bar{z} = a - bi$,
$(a, -b)$ is in the Julia set too.
Work Step by Step
For $z = a + bi$,
if $(a, b)$ is in the Julia set, the absolute value of $z$ will not exceed 2 then.
For the conjugate of $z$,
$\bar{z} = a - bi$,
since complex conjugates have the same absolute value (proved in 8.2 Ex. 64a), therefore, the absolute value of $\bar{z}$ will not exceed 2 also, and hence, $(a, -b)$ is in the Julia set too.
