f[x_]:=(x-1)^2
g[x_]:=2(x-1)+3
Plot[{f[x], g[x]},{x, -3, 5}]

h[x_]:=-x^2+9
j[x_]:=-2x^2+8
Plot[{h[x],j[x]}, {x,-Pi, Pi}, PlotRange->{0, 10}]

Factor[x^2-4x-1]