1. The problem is to explain why the equation $y = x + 9$ is a linear equation. 2. A linear equation in two variables $x$ and $y$ can be written in the form $$y = mx + b$$ where $m$ and $b$ are constants. 3. In this form, $m$ represents the slope of the line, and $b$ represents the y-intercept, which is the point where the line crosses the y-axis. 4. The given equation is $$y = x + 9$$ which matches the form $$y = mx + b$$ with $m = 1$ and $b = 9$. 5. Since the equation can be expressed as $y$ equals a constant times $x$ plus another constant, it represents a straight line. 6. Therefore, $y = x + 9$ is a linear equation because it fits the standard form of a line with slope 1 and y-intercept 9. Final answer: The equation $y = x + 9$ is linear because it can be written in the form $y = mx + b$ where $m=1$ and $b=9$, representing a straight line.