Answer
slope = $-\dfrac{1}{2}$
Work Step by Step
RECALL:
slope = $\dfrac{y_2-y_1}{x_2-x_1}$ where $(x_1, y_1)$ and $(x_2, y_2)$ are the coordinates of the two points on the line.
Use the points on the line $(0, 3)$ and $(6, 0)$ to obtain:
slope = $\dfrac{0-3}{6-0}=\dfrac{-3}{6}=-\dfrac{1}{2}$