x-start or x-finish for 2D graph as a variable?

[cliverlong]

Hi,

Say I have the graph y = 2k and I want to plot this for x between -a and 0 (this is part of STEP I 2011 Q13)

I can enter the equation y = 2k and k is created as a variable.

I then click on "Startup options" and set "Manual / x-start" to "a" rather than a specific numeric value. I set x-finish = 0. Now the default value for any variable = 1, so that may be the problem. Anyway, Autograph replaces "a" to -6 and plots the horizontal line between x=-6 and x=0. I can't seem to make x-start or x-finish a variable.

Is there any way to achieve this?

Regards

Clive