MATLAB: How to write this in MATLAB

MATLAB

Hi guys , Please How I can write

x[n]=3(-1/4)^n u[n] , 1 ≤ n ≤ 20   in matlab..

n=1:20
x=3*(-1/4).^n
stem(n,x)

You can use heaviside function:

 n = -5:1:20;
 x = ((4/5).^n).*heaviside(n);
 stem(n,x)

Bet be aware heaviside(0)=0.5

n=.05:.01:.3;
x_n=(n.^2-1)./(n.^2);
plot(n,x_n);