where the problem .... I want to display the transfer function as a function a ......

clear all, close all, clc;
syms a
%% State space representation
A =  [a 1;-10 -7]
% the state matrix

B = [0;1]
% the input vector

C = [1 0;0 1]
% the output vector
D = zeros(size(C,2),size(B,2))
[num,den]=ss2tf(A,B,C,D)

Answer 1

[Walter Roberson]

Absolutely nothing in the Control Systems toolbox can handle symbolic variables.

The closest you can get is to construct systems with "tunable" values. Tunable values always have a specific current value; they mostly exist to make it easier to change numeric parameters in state space systems.