Checking the sintax of the code only I see 3 issues
1) In the for loop you wrote a(400-x(n)), that is you are addressing the position 400-x(n) = 400 when n = 1 of the variable a. but a is a scalar. so I guess you forgot an operator like, for example a*(400-x(n)) or a+(400-x(n))...
2) you are using sqr instead of sqrt to extract the square root
3) you should preallocate your vectors x and y as x = zeros(i,1); y = zeros(i,1);
Note also that using i as variable can be misleading because it is often used as index