Okay, here we have this:
Considering that the formula for the simple interest rate is:
A = P (1 + rt)
In this case A is equal to $2500, P is the value we need to find, r is the interest rate (in decimal) 0.08, and t is the time, so it's 3 years, replacing we obtain:
2500=P(1+0.08*3)
Now, let's clear P:
2500=P(1+0.24)
2500=P(1.24)
2500/1.24=P
P=2016.13
Finally we obtain that the bank must put $ 2016.13 on a CD to get $ 2,500 in three years.