(a) A point moves on the hyperbola $3 x ^ { 2 } - y ^ { 2 } = 23$ so that its $y$-coordinate is increasing at a constant rate of 4 units per second. How fast is the $x$-coordinate changing when $x = 4$ ?
(b) For what values of $k$ will the line $2 x + 9 y + k = 0$ be normal to the hyperbola $3 x ^ { 2 } - y ^ { 2 } = 23$ ?