a = -1;
b = 2;
g[x_] = (x^2);
h[x_] = 
        g[a] + ((g[b] - g[a])/(b - a)) (x - 
                a);
f[x_] = g[x] - h[x];
TANGENT[x_] = 
        g[(1/2)] + ((g[b] - g[a])/(b - a)) (x - 
                (1/2));
P1 = 
        Plot[{g[x], h[x]}, {x, -1.5, 3}, PlotRange -> {-3, 5}, 
          DisplayFunction -> Identity];
P2 = 
        Plot[f[x], {x, -3, 3}, PlotRange -> {-3, 5}, 
          DisplayFunction -> Identity];
P3 = 
        Plot[{g[x], h[x], TANGENT[x]}, {x, -1.5, 3}, 
          PlotRange -> {-3, 5}, 
          DisplayFunction -> Identity];

Show[{P1, Graphics[{Disk[{a, g[a]},  .05], Disk[{b, g[b]},  .05]}]}, 
        AspectRatio -> 1, 
        DisplayFunction -> $DisplayFunction];

Show[{P2, Graphics[{Disk[{a, f[a]},  .07], Disk[{b, f[b]},  .07]}]}, 
        AspectRatio -> 1, 
        DisplayFunction -> $DisplayFunction];

Show[{P3, Graphics[{Disk[{a, g[a]},  .05], Disk[{b, g[b]},  .05]}]}, 
        AspectRatio -> 1, 
        DisplayFunction -> $DisplayFunction];