Answer
This is a minimum and $m = 3$
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 $~~3.5~\lambda$
This is a minimum and $~~m = 3$
