we know that
If lines f and g are parallel
then
the angle c must be equal to
∠c=[tex]75\°[/tex] ------> by alternate exterior angles
and
∠c+∠d=[tex]180\°[/tex] ------> by supplementary angles
so
∠d=[tex]105\°[/tex]
therefore
the answer is
The angle d should be equal to [tex]105\°[/tex] to prove that f ∥ g
