There are vectorized ways to do this, but using your formula directly:
y = zeros(size(x));
y(1) = something; % <-- you fill this in, e.g. maybe x(1)?
n = something; % <-- you fill this in
for k=2:n % <-- for the rest of the elements through n
y(k) = y(k-1) + x(k); % <-- your formula
end
