How do i code y[n]?

y[n] = x[n] + 0.4x[n-400] + 0.4x[n-800]

Answer 1

[Image Analyst]

Use parentheses instead of brackets:

n = 801; % n must be at least this big.
y(n) = x(n) + 0.4 * x(n-400) + 0.4 * x(n-800);

[Image Analyst]

If you want convolution, use conv().

[Semih Erken]

how to convolve this without conv function

[Image Analyst]

Then I guess it must be your homework assignment. If you're not allowed to use conv(), you'll have to use a for loop.