For this problem a. I could get the right answer with excel. Please help me out which one is problem.

Spencer Enterprises is attempting to choose among a series of new investment alternatives. The potential investment alternatives, the net present value of the future stream of returns, the capital requirements, and the available capital funds over the next three years are summarized as follows: Net Present Capital Requirements ($) Alternative Value ($) Year 1 Year 2 Year 3 Limited warehouse expansion 4,000 3,000 1,000 4,000 Extensive warehouse expansion 6,000 2,500 3,500 3,500 Test market new product 10,500 6,000 4,000 5,000 Advertising campaign 4,000 2.000 1,500 1,800 Basic research 8,000 5,000 1,000 4,000 Purchase new equipment 3,000 1,000 500 900 Capital funds available 10,500 7,000 8,750 a. Develop and solve an integer programming model for maximizing the net present value. b. Assume that only one of the warehouse expansion projects can be implemented. Modify your model from part a. C. Suppose that if test marketing of the new product is carried out, the advertising campaign also must be conducted. Modify your formulation from part b to reflect this new situation x Solver Parameters 4000 6000 x4 10500 4000 x6 8000 3000 Obj Coef DV Obj Func. Set Objective: $B$4 1 To: O Max O Min O Value Of: 0 Subject to ci C2 C3 3000 1000 4000 2500 3500 3500 6000 4000 5000 2000 1500 1800 5000 1000 4000 LHS 1000 500 900 0