** Using WeBWorK **

Hello,

I am making a differential equations problem -second order homogeneous. I am unable to represent y", y' in Formula objects in order to make something like:
$DiffEqu = Formula("y'' - ($a + $b)y' + ($a*$b)y")->reduce.

Is there a way of displaying a variable as something else? -declaring say, x, y, and z as variables but displaying x and z as (y') and (y'') respectively, possibly using {alias}.

Or is there a way of simply adding y'' and y' as legitimate variables?

Thank you,
-Spyro Roubos