You just need to remember the types of stationary (turning) points
$-maximum: when$\,\,\frac{dy}{dx} = 0\,\,$and$\,\,\frac{d^2y}{dx^2}<0
$-minimum: when$\,\,\frac{dy}{dx} = 0\,\,$and$\,\,\frac{d^2y}{dx^2}>0
$-inflexion: when concavity changes ie$\,\,\frac{d^2y}{dx^2}=0
$Now a horizontal...