The Miller indices are found by taking reciprocals of the intercepts in terms of lattice constants:
\[
\frac{a}{3a}, \frac{b}{-2b}, \frac{c}{(3c/2)}
\]
This gives:
\[
\left(\frac{1}{3}, -\frac{1}{2}, \frac{2}{3} \right)
\]
Multiplying by 6 to clear fractions, we get:
\[
(2, 3, 4)
\]