If you notice that the differences between the outputs are:
43-25=18
61-43=18
79-61=18
The rule could very well be:
f(n)=18n+C
Substitute n=0
f(0)=18*0 + C = 25
so C=25
so the rule is
f(n)=18n+25
Check f(1), f(2), f(3) to make sure that the rule is correct.