Answer
Destructively
Work Step by Step
We know that
$ l_2-l_1=m\lambda $
We plug in the known values to obtain:
$143m-78m=m(26.0m)$
$65m=m(26.0m)$
$ m=2.5$
Now $ l_2-l_1=(m-\frac{1}{2})\lambda $
$\implies l_2-l_1=(3-\frac{1}{2})\lambda $
The last equation shows the form of destructive interference. Thus, the waves interfere destructively.
