Seif, I believe this is what you are trying to do:
%calculating velocity
V3 = 10; % velocity of the piston in cm/sec
%global c
th2 = pi/6; % initial value for th2
th3 = pi/6; %initial value for th3
% c= 28.8:0.2:52.6;
C = 28.8:0.2:52.6;
for i = 1:numel(C)
% c = C(i);
th3(i) = acos((-1425 - (C(i)).^2)./(110.*C(i)))*(180/pi);
th2(i) = acos((C(i).^2 - 4625)./(-4400))*(180/pi);
M = [-40*sin(th2(i)) C(i).*sin(th3(i)); 40*cos(th2(i)) -C(i).*cos(th3(i))];
N = [V3*cos(th3(i)); V3*sin(th3(i))];
W(:,i) = M\N;
end
W2 = W(1,:);
W3 = W(2,:);
figure;
subplot(2,1,1); plot(th2,W2,'r','linewidth',2);grid
xlabel('{\theta_{2}} [deg]');
ylabel('{\omega_{2}} [rad/sec]');
subplot(2,1,2); plot(th2,W3,'r','linewidth',2);grid
xlabel('{\theta_{2}} [deg]');
ylabel('{\omega_{3}} [rad/sec]');
Also, I recommend replacing the loop index name i by ii. i is also the imaginary unit in MATLAB.
