find a general term for the sequence

a1,a2,a3,a4.....
15,23,31,39

notice that they increase by 8

so
t(n) = 8n + b
when n=1 we want that to be 15
15 = 8+b
b = 7

term(n) = 8n + 7

check :
t(4) = 8(4) + 7 = 39 , looks good!