For which value of $c$ does the line $3x+4y+c=0$ pass through the origin?

Two special cases:

**$y$ = constant**: a horizontal line (parallel to $x$-axis). Example: $y=4$ is the line of all points with ordinate 4. Every solution has $y=4$ regardless of $x.\ast \ast x$ = constant**: a vertical line (parallel to $y$-axis). Example: $x=-2$ is the line of all points with abscissa −2.

Algebraically: $y=4$ is $0x+y-4=0$; $x=-2$ is $x+0y+2=0$. They fit the standard form ax + by + $c=0$ with one of a, b being 0.