# Subtracting from an investment each month question

So I have this maths question which I am struggling to solve as it seems to be a backwards compound interest or possibly an annuity question. I was wondering whether anyone might be able to provide a hint as to how I can do this question as from what I can tell you have to calculate the total amount of money you would have used at the end of 18 years after investing the money yet taking out payments every time. I am just unsure how to go about doing this and any help would be greatly appreciated.