we have the sequence
-108, -91,-74,-57,...
we have
a1=-108
a2=-91
a3=-74
a4=-57
step 1
Find the common difference d
a2-a1=-91-(-108)=17
a3-a2=-74-(-91)=17
so
d=17
step 2
thw general equation for an arithmetic sequence is equal to
an=a1+d(n-1)
we have
d=17
a1=-108
substitute
an=-108+17(n-1)
an=-108+17n-17
an=-125+17n
therefore
the answer is an=-125+17n
