The pyramid is bounded by planes: $x=0$, $y=0$, $z=0$ and $9x-y-3z=54$. IT is needed to calculate the volume $V=\frac {1} {3} bh$, where where $b$ is the area of the base and $h$ the height from the base to the apex.
I am not sure how to find the coordinates of the $4$ vertices to calculate the area of the base and $h$.
Any hints?
On
One vertex is at the intersection of the line shared by the two planes $x=0$ and $y=0$ and the line shared by the two planes $x=0$ and $9x-y-3z=54$. So if you solve the three simultaneous equations $$ x=0\\y=0\\9x-y-3z=54 $$ you will get one vertex of the pyramid to be $(0,0,-\frac{54}3)$.
By the way, once you have the other vertices it will be much easier to use the base in one of the three coordinate planes, and then use $\frac13 bh$ where $b$ is the are of the base and $h$ is the "height" along the third coordinate, than to use the base as the three vertex points other than $(0,0,0)$ and try to figure out the height by finding the perpendicular distance of that plane to the origin.
Three of the planes meet at $x=y=z=0$. Three of the planes meet at $x=y=0, 9x-y-3z=54$. Three of the planes meet at $x=z=0, 9x-y-3z=54$. Three of the planes meet at $y=z=0, 9x-y-3z=54$.
This gives the coordinates: $$(0,0,0), (0,0,-18), (0,-54,0), (6,0,0)$$ respectively.