I=$26.25i=$26.25, p=$500p=$500, t=18 t=18 months

Next time, please include the directions for the problem you post.  Here it appears that you have given the values of I, P and t two times each.  
Unfortunately, I have to guess what you're looking for.
 
Assuming that I=$26.25 is the interest earned on Principal P=$500, and that t is the length of time over which the Principal earns interest,

I=Prt.  With I, P and t given, it's obvious that our job is to find the annual interest rate, r.      So, from I=Prt, we get

$26.25 = $500 (r) (1.5 years)            Solve this for the interest rate, r.  
Express r both as a decimal fraction and as the equivalent percentage.