Answer:

Steps:
A decrease of an amount x by 63% is the same as (x - x*0.63), or x*(1-0.63), or x*0.37.
The decrease is per year. So after one year the amount is 0.37*x, which then again decreases by 63%, so the amount after two years will be
, and so on. And so after n years, the amount will be:

Finally, we are given the amount x - the initial number of whale sharks 25000, so the final exponential function describing the whale shark count after n years:
