C is the compounded amount
P is the principle (starting) amount
p is the rate (what you're after)
n is time.
Given that it is done quarterly for 5.25 years,
Let's do some algebra with our n value as 21.
Multiply out by P
Take the 21st root of both sides.
multiply both sides by 21, then subtract 21
Plug in the values
That should find you your rate.
x = 21((1.25)^(1/21)) -21 - Wolfram|Alpha
p = 22.43%