Thanks so much - that worked for me.  I have one last request - is there a way to add minor axis lines on the graph to make the crossover area a little easier to see.  This would look similar to what Excel does, in that you can specify the number of axis gridlines on the graph.

Almost exactly what I need.  I did get the following error:

"Error, (in unknown) invalid input: lhs received filledregions, which is not valid for its 1st argument, expr"

Is there a way to specify a red color for >423e-6 and green for <=423e-6?

Thanks in advance

The eq should be 1/2*L*(14.4/(1.5+R))^2<=423e-6