Question

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

Answer 1

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