Parametric equation in matrix form

Hi,

I am trying to solve a parametric equation in matrix form. I have to multiply and evaluate a number of matrices symbolically.

I have to calculate a force Nx which can be applied so that failure occurs according to certain criterion.

This means I need to solve the matrices and equations in parametric form and obtain the stress matrices in terms of the applied load Nx.

I am however not being able to solve the equations symbolically since I get errors like 'the value name must be the name of a function' or ' the array should be compatible'  or 'the variable is undefined, check the label is set correctly'. Please advice how I can proceed.

Thanks