How to calculate monthly instalment specifed in task in maxima?

Question

I am stuck trying to make a solution for a task my tutor asked me to do using wxMaxima.

Here's the task: "On June 1, you take a mortgage that is repaid in fixed monthly instalments for 10 years at the end of each month (we assume that there are 12 equal months in a year). The loan amount is 130 000.

The loan interest rate is variable, i.e. the bank changes it every set interval. Please calculate the amount of the fixed loan instalment (as described above).

Then, assuming that on the first day of March, your bank decreases the interest rate on your loan by 2 percentage points.

Please calculate the new instalment. The current interest rate should be 9.49% and assumed as capitalised annually. Also I have some formulas for annuity:

AIFV(A,r,t,m):=A*sum((1+r/m)^i,i,0,t*m-1);
AIPV(A,r,t,m):=A*sum((1+r/m)^(-i),i,1,t*m);
ADFV(A,r,t,m):=A*sum((1+r/m)^i,i,1,t*m);
ADPV(A,r,t,m):=A*sum((1+r/m)^(-i),i,0,t*m-1);,

where A is installment, r is interest rate, t is years and m is how often it is capitalized.

I've tried to do it, but I don't know how to handle the change in interest rate in next periods.

I've made a file in Excel calculating this, but my tutor said that excel is not enough and I need to do it in wxMaxima.

Can anyone help or just give me a hint?

Thanks in advance!