Let's look at your code section by section.
u = @(t) (t>=0);
r = @(t) t.*u;
This will not work as written. You cannot multiply a number and a function handle. What you can do instead is to multiply a number and the result of evaluating a function handle.
r = @(t) t.*u(t);
In the next section there is a similar problem.
x = @(t) r(t)-r(t-1)-3*u(t-1)-2*r(t-3)-r(t-4)-r(t-5)-u(t-7);
t = [-2:0.01:7];
plot(t,x)
The plot function will not accept a function handle as its second input, but it can accept the result of evaluating a function handle.
plot(t, x(t))
