What is an example of a student handbook rule that has a necessary oxford comma?

Here’s an example sentence to help show why I think the Oxford comma should always be used, wherever possible: -

- I like Ziggy Stardust and The Spiders from Mars, and Tom and Jerry.

This could be ordered, as suggested above for non-advocates, to:-
- I like Tom and Jerry, and Ziggy Stardust and The Spiders from Mars.

But may not be reordered to: -

- I like Ziggy Stardust, Tom, Jerry and The Spiders from Mars. 

x=4, y=4

step-by-step explanation:

y=2x-4;

x+2y=12

we can use substitution since the first equation is solved for y

where ever you see y in the second equation, replace it with 2x-4

x + 2(2x-4) = 12

distribute the 2

x+4x-8 = 12

combine like terms

5x-8=12

add 8 to each side

5x-8+8=12+8

5x=20

divide by 5

5x/5 = 20/5

x =4

we still need to solve for y

y = 2x-4

we know x =4

y = 2(4) -4

y = 8-4

y=4