 <p>I believe this does what you want:</p> <pre><code>SlopeInterceptDemonstration[{mmin_, mmax_}, {bmin_, bmax_}] := Manipulate[ Plot[m*x + b, {x, xmin, xmax}, AspectRatio -&gt; 1, PlotRange -&gt; {xmin, xmax}], {{m, mmin, "m"}, mmin, mmax, 0.1}, {{b, bmin, "b"}, bmin, bmax, 0.1}, {xmax, None}, {xmin, None}, Initialization :&gt; {xmax = Max[Abs[bmin + mmin], Abs[bmax + mmax]]*1.2, xmin = -xmax} ] </code></pre> <p><code>{xmax, None}</code> is used to localize <code>xmax</code> in the Module. The method with <code>DynamicModule</code> shown in the other answer is standard and more flexible.</p>