Hi Sam, what you have are two back-to-back annuity problems.
The first one is the future value of an ordinary annuity, "FVOA." For it, you have the monthly deposit amount, the interest rate, the compounding frequency (12) and the number of years (21), which will translate into the number of periods (21 x 12 = 252). In this first equation, we are solving for FVOA.
The second annuity problem - first, we need to recognize that your FVOA, above, is the total amount you have available for withdrawal starting 21 years from now. So, it is the present value of an annuity, "PVOA," (I assume ordinary and not due) starting 21 years from now. For it, we have PVOA, the number of years (26) and withdrawal frequency (12), which will translate into the number of periods (26 x 12 = 312). In this second equation, we need some input for the interest rate. We can otherwise assume none / we can assume a similar rate as for the first equation / we can assume, let's say, the average Fed rate for the last 30 years. Unless the money (FVOA) is sitting in a no-interest checking account, it will continue to accrue some interest income because not all of the FVOA is withdrawn at once.
Once you have set or assumed an interest rate, you can THEN solve for the monthly withdrawal amount in the second equation.