I've got a question that looks like this:
Use the normal gradient vector to determine the equation of the line/plane tangent to the given curve/surface at point P.
$x^4 + xy + y^2 = 19$, P(2,-3)
I know how to use the gradient formula, and end up with an answer of $29x - 4y = ?$. The answer given in my book is 29x + 4y = 70. Where is this 70 coming from? I'm having a lot of trouble trying to figure it out.