Chapter 1 - First-Order Differential Equations - 1.3 The Geometry of First-Order Differential Equations - Problems - Page 32: 4

$\frac{dy}{dx}=-\frac{c}{x^2}$

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.