Here's how you might calculate and plot some similar curves. You can take the syntax and apply it to yours.
L = 10;
x = -1000:0.01:1000;
u = (10^-3)*x - 1800*(x.^2-3*L*x);
plot(x,u)
hold on
u = (10^-3)*x - 2*(x.^2-4*L*x) + 6.6*(x.^3-2*L^2*x) ;
plot(x,u);