using "solve" and getting rid of arbitrary parameters

I am trying to solve a system of equations whose answer will depend on arbitrary values.

For example, consider the following system of 2 equations and 3 unknowns:

syms x1 x2 x3
E=[x1+2*x2+x3;x3-x1+x2];
a=solve(E==0,x1,x2,x3)
A=[a.x1 a.x2 a.x3]

this gives me an answer that depends on an arbitrary value z. (infinitely many solutions).

I would like MATLAB to directly give me an answer (with no arbitrary values). So for instance, I would like to get an answer where the x1 value is positive. Is there a way to do that?

I am using "solve" in a subroutine, and after that, I would like to test the answer to see if it is a non negative array. But whenever the code reaches the answer, it tells me there is an error (obviously) because of the unknown parameter.