Answer
This is an intermediate state closer to a maximum and $~~m = 2$
Work Step by Step
A maximum has a path length difference: $~~\Delta L = m~\lambda$
A minimum has a path length difference: $~~\Delta L = (m+\frac{1}{2})~\lambda$
It is given that the path length difference is $~~1.8~\lambda$
This is an intermediate state closer to a maximum and $~~m = 2$
