Answer
Please see the work below.
Work Step by Step
(a) We know that $ m=\frac{l_2-l_1}{\lambda}$
We plug in the known values to obtain:
$ m=\frac{130m}{26m}=5$
We have obtained the value as an integer, hence the interference is constructive.
(b) We know that
$ m=\frac{l_2-l_1}{\lambda}$
We plug in the known values to obtain:
$ m=\frac{91.0cm}{26.0cm}=3.5$
Since the value we have obtained contains a decimal point, the interference is destructive.
