Been studying GP and been given this as homework - I'm at a loss as to how this is done...

A mortgage of 80k is taken and is to be repaid over 25yrs. Interest is charges at the begining of each year on the outstanding debt at a rate of 7%. Repayments are to be made monthly begining in the first month.

(i) Supposing each repayment is x. Write down the expression of the outsatnding debt at the end of each of the first three years. In particular,show that the debt after three years is given by:

80000(1.07)^3 - 12x(1 + 1.07 + (1.07)^2)

(ii) Obtain the expression for the debt after 25yrs and use this expression to calculate the monthly repayments.

(iii) Calculate, as a percentage, how much of the mortgage is paid off in each of the first two years.

Youyr help is massively appreciated on this one!