The link you posted isn't really working so I'm just going to assume that the function you're referring to is the standard formula for compounded interest, that is p(1+r)^x. In this case the principal amount is $1000 and x is 3. So it would look something like 1000(1+r)^3. Well in order to find the rate of annual interest if the balance after three years is B you simply have to set the formula equal to B and then isolate r.
1000(1+r)^3=B
First divide both sides by a 1000,
1000(1+r)^3=B (1+r)^3=B/1000
then cube root both sides,
(1+r)^3=B/1000 1+r=cuberoot(B)/cuberoot(1000) 1+r=cuberoot(B)/10
and then simply subtract 1 from both sides getting the final equation r=(cuberoot(B)/10) -1, which is the formula for finding the value of r given the value of B
+22 
