Given $z = f(x, y)$, $x = x(u, v)$, $y = y(u, v)$, with $x(2, 6) = 4$ and $y(2, 6) = 4$, calculate $z_u(2, 6)$ in terms of some of the values given in the table below.
$f_x(2, 6) = p$, $f_y(2, 6) = a$, $x_u(2, 6) = r$, $y_u(2, 6) = b$
$f_x(4, 4) = s$, $f_y(4, 4) = q$, $x_v(2, 6) = l$, $y_v(2, 6) = 3$