# What is the slope of the tangent line of  x/(x-3y) = C , where C is an arbitrary constant, at (-2,1)?

$y = \frac{1 - \frac{1}{C}}{3} x$. As C is fixed as 2/5, by setting (-2, 1) on the line,
the equation is $y = - \frac{x}{2}$. Its slope is $- \frac{1}{2}$.