f=3*x-x^2

f

Solve[f==0,x]

data={{1,2},{2,4},{3,4}}

l=ListPlot[data]

p=Plot[f,{x,0,4}]

Show[l,p]

fit=Fit[data,{1,x,x^2},x]

ff=Plot[fit,{x,0,4}]

Show[ff,l]