Manipulate[
Plot[{(1/2) k x^2 + (1-k) x + (1/2) k, x }, {x, 0, 2}, PlotRange -> {0,2}, AspectRatio -> Automatic, PlotStyle -> {{Thick, Black}, {Thick, Red} }, Ticks -> {Range[0,2,1/2], Range[0,2,1/2]}], {{k,0}, -20, 20}]