You write:
tau = (mu/(2*delta_y))*(4*un,2-un,3)
But what des that mean to you? Is the fragment
(4*un,2-un,3)
intended to be a vector of length 3? Then you need to use square brackets to create a vector. So you would write:
tau = (mu/(2*delta_y))*[4*un,2-un,3];