What is the value of y in the exponential decay function when x equals 2

Y=ar^t  where 0<r<1 for exponential decay to occur, from your question it is unclear what x is.  Perhaps it represents time?

y=ar^x  

To know what the value of y is, you would have to know the rate,r, and the initial amount, a, is.  As it is all you have is:

y=ar^5