2 + 2

Plot3D[x (x - y) (x + y), {x, -3, 3}, {y, -3, 3}, 
      BoxRatios -> {1, 1, 2}]

Plot3D[x y (x - y) (x + y), {x, -3, 3}, {y, -3, 3}, 
      BoxRatios -> {1, 1, 2}]

Table[
        Plot3D[(x - a y) (x - y) (x + y), {x, -3, 
            3}, {y, -3, 3}, BoxRatios -> {1, 1, 2}, 
          PlotRange -> {-20, 20}], {a, -1, 
          1,  .1}];

Plot[Sin[x]/x, {x, -3 Pi, 3  Pi}]

Plot[((1 - Cos[x])/(x^2)), {x, -3 Pi, 3  Pi}]