MATLAB: How we plot the the below function in MATLAB

plottingsolve

x₃[n]= (0.8)^n U[n] -10< n <10

See if this does what you want:

n  = -19:20;
Un = [zeros(1,10) ones(1,20) zeros(1, 10)];
x3 = 0.8.^n .* Un;
figure(1)
stem(n, x3)
grid

I have no idea.

I would just do something like this, and hope for the best with respect to whatever that exp() call is supposed to do:

u = @(t) t>=1;
x = @(n) n.*(u(n)-u(n-10) + 10*exp(-0*(n-10)) .* (u(n-10)-u(n-20)));
n = linspace(0, 20, 1000);
figure
plot(n, x(n))
grid

Make appropriate changes when you resolve the problem with the exponential term.

Read about gridata and scatteredInterpolant.