1. The problem states there is a proportional relationship between the age of Brenna's apple tree $x$ (in years) and its height $y$ (in feet). 2. In a proportional relationship, the formula is: $$y = kx$$ where $k$ is the constant of proportionality (the slope). 3. From the graph description, the line passes near $(0,0)$ and at $x=10$, $y \approx 10.5$. 4. To find $k$, use the formula for slope: $$k = \frac{y}{x} = \frac{10.5}{10}$$ 5. Simplify the fraction: $$k = \frac{\cancel{10.5}}{\cancel{10}} = 1.05$$ 6. Therefore, the constant of proportionality is $1.05$ feet per year. Final answer: $k = 1.05$