Chapter 3 - Introduction to Graphing - 3.3 Graphing and Intercepts - 3.3 Exercise Set - Page 180: 91

The graph of $x+y=C$ is a line that falls from left to right whose x-intercept is $(C, 0)$ and whose y-intercept is $(0, C)$.

For whatever value $C$, the graph of the equation $x+y=C$ will have an x-intercept $(C, 0)$ and a y-intercept of $(0, C)$. Additionally, the graph will be a line that is falling from left to right.