## Differential Equations and Linear Algebra (4th Edition)

$\frac{dy}{dx}=-\frac{c}{x^2}$
Take the derivative of the function in respect to $x$ (convert $c/x$ to $cx^{-1}$ and use power rule) to get $$\frac{dy}{dx}=-\frac{c}{x^2}$$ This $\frac{dy}{dx}$ is the slope of the tangent line.