geometry app question...

[webmasterpdx]

I'm trying to draw a circle, tangent to 3 lines using incircle().

I have 3 lines defined as:
GA: line(y=(2*x+5))
GB: line(y=(-2*x-4))
GC: line(y=(-7*x+5))
and then I tried incircle as in:
GD: incircle(GA,GB,GC)

It draws the lines fine, but there is no circle on the plot screen....
What am I doing wrong?
Thanks
-Donald