Extreme problems with solver! Cannot find Linear resolution...

Hi i am having extreme problems with solver. It is for an assignment for uni and for the life of me i cant work out where i have gone wrong.

The whole point of the assignment is that the problems are liner.. therefore any answer without selecting the box "Assume linear model" is deemed not the best option (although it may be correct)

Although this problem is not hard compared to others, to explain it all here would be quite difficult for me. So i have included the Question sheet (It is question number 3) and i have also included the Worksheet that i have compiled.

Im not asking for the answers to this, i simply want to know why something is not LINEAR in my setup... or whats causing this..this has been 5 days of trying to work this out!!

Any help would be greatly appreciated.
Cheers
Andy