For $2x+y=9$, what is the value of $x$ when $y=4$?

Pick any value for one variable; solve for the other. Repeat to get many solutions.

Example: $3x+2y=12$.
- Let $x=0:2y=12$ → $y=6$. So (0, 6) is a solution (this is the $y$-intercept).
- Let $x=2:6+2y=12$ → $y=3$. So (2, 3) is a solution.
- Let $x=4:12+2y=12$ → $y=0$. So (4, 0) is a solution (this is the $x$-intercept).
- Let $y=0:3x=12$ → $x=4$. Same as above.

Four solutions found in seconds, and infinitely more available.