MATLAB: Plotting trajectories of a system of equations.

differential equationsphase portraits

Hi all,
Im writing a project and have been told that MatLab is the best way to visualise what is happening, however i am very new to matlab.
the two equations I wish to plot are simple, dx/dt = ax and dy\dt = -y. I want to vary a and then see how the phase portrait changes by plotting some trajectories and showing how the fixed point at the origin changes according to the value of a. I have solved the system having no problems but them when it comes to plotting anything i am having difficulties.
The code i have so far is:
syms x(t) y(t);a=1;A = [a 0; 0 1];Z = [x;y];odes = diff(Z) == A*Z[xSol(t), ySol(t)] = dsolve(odes);xSol(t) = simplify(xSol(t))ySol(t) = simplify(ySol(t))xdom = linspace(-10,10,100);ydom = linspace(-10,10,100);U = a.*x;V = -1*y;[X,Y] = meshgrid(xdom,ydom);quiver(X,Y,U,V)% this returns an error saying unable to conver expression into double array 

syms x(t) y(t);a=1;A = [a 0; 0 1];Z = [x;y];odes = diff(Z) == A*Z[xSol(t), ySol(t)] = dsolve(odes);xSol(t) = simplify(xSol(t))ySol(t) = simplify(ySol(t))xfcn = matlabFunction(xSol)                             % Create Anonymous Function From Symbolic Functionyfcn = matlabFunction(ySol)                             % Create Anonymous Function From Symbolic Functionxdom = linspace(-10,10,100);ydom = linspace(-10,10,100);[X,Y] = meshgrid(xdom,ydom);U = a.*X;                                               % Use ‘X’ To Calculate DerivativesV = -1*Y;                                               % Use ‘Y’ To Calculate Derivativesquiver(X,Y,U,V,5)                                       % Scale Arrows At 5axis([-1  1    -1  1])                                  % Zoom In To View Details