working out if an investment is economic. (question inside)

if an investment returned $1000 every year for the first 4 years and returned $1200 yearly after that for 5 years. which after the return becamse $2000 forever. The investment requires three instalments of the same size; immediately, in two years time and five years time. the interest rate is constant and 10%

What is the maximum size of instalments before the investment becomes uneconomic?

Can some one tell me what formulae to use for what part of this question as i am completely stumped. any help GREATLY APPRECIATED.