Given: (x is number of items)
Demand function: d(x)=4000.4x\displaystyle {d}{\left({x}\right)}={400}-{0.4}{x}
Supply function: s(x)=0.4x\displaystyle {s}{\left({x}\right)}={0.4}{x}

Find the market demand:  

Find the producers surplus at the market demand: