7th term is 15 and the fourth term is 9
Common difference: (15-9)/(7-4) = 2
Counting backwards from the fourth term will give us the "zeroth term"
A0 = 1
The closed formula for the sequence is
An = dn + A0
The 25th term is:
A25 = 2(25) + 1
A25 = 51
Hope this helps!