DynamicModule[{k}, Panel[
  Column[{
    Row[{"a", " ", Slider[Dynamic[k], {1, 9, 0.3}]}], 
    With[{a = k}, 
     Dynamic@ContourPlot[t + Sin[w t] == a, {t, 0, 10}, {w, -5, 5}, 
       PlotPoints -> 25, ImageSize -> 300, 
       PlotLabel -> "w vs.t given a", FrameLabel -> {"t", "w"}
       ]]}]
  ]
 ]
