example 10.1 p.239

y1 = 
      a x^2 + b x + c /. {a ->  1, b ->  2, 
          c ->  -3}
Plot[y1, {x, -5, 5}]

example 10.2  p.239

<< Graphics`ImplicitPlot`

eq2 = (x/a)^2 + (y/b)^2 == 1 /. {a ->  2, 
          b ->  1}
ImplicitPlot[eq2, {x, -2, 2}]

example 10.3  p.240

x3 = a Sin[θ] /. {a ->  1}
y3 = 
      a Cos[n θ] /. {a ->  1, 
          n -> 3}
ParametricPlot[{x3, y3}, {θ, 0, 2 π}]