Dear Ahmed, here is the code:
Lgo = 1; % constant value for Lgo
Lg2 = 1; % constant value for Lg2
theta = -pi:0.01:pi; % time series -pi to pi
M = -0.5 * Lgo + Lg2 * cos(2 * theta - 120);
plot(theta, M) % plot of M vs. theta
I hope it helps. Good luck!