Dear Edo,

the Demonstration project comes with the full code, so that you can see how it is done. You only need to adapt it to your problem and you get something like:

Manipulate[
 Plot[((a/(1 - b)) + (b/(1 - b))*c), {c, 0, 100}, 
  PlotRange -> {{0, 100}, {0, 300}}, AxesOrigin -> {0, 0}, 
  LabelStyle -> Directive[Bold, 14], AxesLabel -> {"Xaxis", "Yaxis"}, 
  Ticks -> {{{c/2, "Q"}}, {{
      ((a/(1 - b)) + (b/(1 - b))*c/2), "P"}}}, 
  Epilog -> {{Thick, Dashed, 
     Line[{{c/2, 
        0}, {c/2, ((a/(1 - b)) + (b/(1 - b))*c/2)}}]}, {Thick, Dashed,
      Line[{{c/
         2, ((a/(1 - b)) + (b/(1 - b))*
           c/2)}, {0, ((a/(1 - b)) + (b/(1 - b))*c/2)}}]}}], {{a, 30},
   0, 50}, {{b, .6}, .0000001, 1}, {{c, 90}, 0, 100}]

which I hope does what you want.

Cheers,

Marco