How to define varying material property along the length of the material

Hi, I am new a new comsol user. I want to define a material property which will change along the length of the material. say, E=a*X+b. where, E is the stiffness of the material, a & b are constants, X defining the longitudinal direction of the material.

Can anyone advice me, how to define such equation based material property in a material.

Regards