an=a1+d(n-1)
a1=first term
d=common difference
n=which term
a1=7
a2=3
a2=3=7+d(2-1)
3=7+d(1)
3=7+d
-4=d
an=7-4(n-1)
expanded
an=7-4n+4
an=11-4n
domain is the numbers you can use for n (1,2,3,4,...)
the range is the output or the numbers you get in the sequence (7,3,-1,-5,...)
for the domain, we have to use numbers like 1,2,3,4,5
no 0
so we use natural numbers, that is the domain