Which of the statements given below are correct?

$P$ - A standard equation in which a variable is squared will never make a straight line.

$Q$ - The slope of the line $ax + by + c = 0$ is given as ($\frac{-a}{b}).$

$R$ - The slope of a line is always defined as either positive, negative or zero.

$S$ - The vertex of a parabola may represent the maximum or the minimum value of the function.