Call the plot() command first and then define the labels and legend -
syms x s
u1 = stepfun(x,0);
f1 = x.*exp(-3*x).*u1;
Lf = laplace(f1, s);
disp(Lf);
t = (-1:0.1:1);
u = stepfun(t,0);
f = t.*exp(-3*t).*u;
figure(2);
%% plot command
plot(t,f);
%% add details after plotting
xlabel('Time,(s)')
ylabel('f(t)')
grid;
legend('f(t)')
