I dont understand my error. Invalid expression. Check for missing multiplication operator, missing or unbalanced delimiters, or other syntax error.

syms x y z 

a = (1)/((x*10x10^3)*y*(z + 10x10^-9)) == 3.9*10^9;

b = (1.)/(z+10x10^-9)*(((10x10^3)*y)/((10x10^3)+y)) == 1.571*10^4;

c = (10x10^-9)/(x*y*(z + 10x10^-9)) == 1.571*10^4;

solve (a,x);
solve (b,y);
solve (c,z);


[A,B] = equationsToMatrix([a, b, c], [x, y, z]);
X = linsolve(A,B);

What should i change? 

[Torsten]

10x10^3 ?? 10x10^-9 ?? What does that mean ?

It means:

Invalid expression. Check for missing multiplication operator, missing or unbalanced delimiters, or other syntax error.

[Walter Roberson]

you cannot use x for multiplication in MATLAB. You should mostly use the .* operator