well this problem can be solved with Algebra
the rule here is that for every incrementing sequence you increase your number by 6.
T(n)=18+6(n-1)
for 603 to be in the sequence, it has to male sense with the function above
603=18+6(n-1)
603-18=585
585=6(n-1)
n-1=(585/6)
n=(585+6)/6
n=591/6
here is your answer.
