Q.What is the slope of

A horizontal line has zero slope (the Y-variable never changes); a vertical line has an undefined (infinite) slope (the X-variable never changes).

1. Horizontal line. Here $y$ stays the same for every value of $x$, so $\Delta y = 0$, giving $m = \dfrac{0}{\Delta x} = 0$. Economic example: a perfectly elastic supply curve, where sellers supply any quantity at one fixed price — a horizontal line at that price.
2. Vertical line. Here $x$ stays the same for every value of $y$, so $\Delta x = 0$, making the slope formula $m = \dfrac{\Delta y}{0}$ undefined (division by zero). Economic example: a perfectly inelastic supply curve, where the same fixed quantity is supplied whatever the price — a vertical line at that quantity.
   ✓Final answer

   Horizontal line: slope = 0 (perfectly elastic supply, one example). Vertical line: slope = undefined (perfectly inelastic supply, one example).

Saying a vertical line has 'zero slope' — it is the horizontal line that has zero slope; a vertical line's slope is undefined, not zero, because it involves dividing by zero.