We'll have to assume that interest is charged annually.
interest = i = p*r*t, where p is the initial amount ($25000), r is the annual interest rate as a decimal fraction, and t is the length of time, in years.
Then $2625 = $25000*0.035*t. Solve for t:
$2625 ------------ = 0.035t = 0.105. Dividing both sides by 0.035, we get $25000
There is also an exponent of '6' outside. According to exponential properties, when an exponent is within an exponent, you multiply them together. Therefore: