Find the number of square units bounded by the $x$-axis, the $y$-axis and the line $x+ y= 6$.
I recognize that this is a problem about finding the area of a region in the coordinate plane. The line $x+y=6$ is a straight line with a slope of $-1$ and a $y$-intercept of $6$. The region bounded by the line and the axes is a right triangle with vertices at $(0,0)$, $(0,6)$, and $(6,0)$. I can use the formula for the area of a triangle, which is $\frac{1}{2}bh$, where $b$ is the base and $h$ is the height. In this case, the base and the height are both $6$ units, so the area is $\frac{1}{2}(6)(6)$. This simplifies to $18$ square units.

