text: ## Geometry: Common Core (15th Edition)
  
  $m = -1$
  We are given the two points X $(0, a)$ and Y $(-a, 2a)$. Let's use the formula to find the slope $m$ given two points: $m = \frac{y_2 - y_1}{x_2 - x_1}$, where $(x_1, y_1)$ and $(x_2, y_2)$ are points on the line. Let's plug in the values into this formula: $m = \frac{2a - a}{-a - 0}$ Simplify the numerator and denominator: $m = \frac{a}{-a}$ Divide both the numerator and denominator by their greatest common factor, $a$: $m = -1$