Manipulate[
  Plot[{15-x+income-45+pop,17-x,x,x-comp+tax},{x,0,25},PlotRange->{{0,20},{0,25}},
  AxesLabel->{"millionsofpoundsofsugarsold","priceperpound"},
  PlotStyle->{Directive[Red,Dotted],Directive[Red,Thick],
  Directive[Blue,Thick],Directive[Blue,Dotted]},ImageSize->{500,300}],
  {{income,27,"AverageConsumerIncome"},9,30,0.5,Appearance->"Labeled"},
  {{pop,28,"Consumerpopulation"},10,50,0.5,Appearance->"Labeled"},
  {{comp,7,"#offirmsintheSugarMarket"},1,10,0.5,Appearance->"Labeled"},
  {{tax,2,"TaxleviedonFirmsinSugarMarket"},0,20,0.5,Appearance->"Labeled"}]