IIRC

Where,

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.

Original Equation

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%