If $$2x=4y=z$$, what is $$x - y$$ in terms of $$z$$?

Correct.
[[snippet]]
Your **goal value** is what the question asks for:
>$$x-y = 4-2 = \color{blue}{2}$$.
Replace $$z$$ with 8 in the answer choices, and eliminate any answer choices that don't equal 2:
>A) $$~ \frac{z}{4} = \frac{8}{4} = \color{blue}{2}$$. Don't eliminate.
>B) $$~ \frac{z}{2} = \frac{8}{2} = \color{red}{4}$$. Eliminate.
>C) $$~z=\color{red}{8}$$. Eliminate.
>D) $$~2z=2(8)=\color{red}{16}$$. Eliminate.
>E) $$~4z=4(8)=\color{red}{32}$$. Eliminate.
Answer choice A is the only one that matches the goal value, and so it must be the right answer choice.

Incorrect.
[[snippet]]
Your **goal value** is what the question asks for:
>$$x-y = 4-2 = \color{blue}{2}$$.
If you try $$z=8$$ in this answer choice, it will not match your goal value.
>$$ \frac{z}{2} = \frac{8}{2} = \color{red}{4}$$.
Eliminate.

Incorrect.
[[snippet]]
Your **goal value** is what the question asks for:
>$$x-y = 4-2 = \color{blue}{2}$$.
If you check $$z=8$$ in this answer choice, it doesn't match your goal value.
>$$z = \color{red}{8}$$.
Eliminate.

Incorrect.
[[snippet]]
Your **goal value** is what the question asks for:
>$$x-y = 4-2 = \color{blue}{2}$$.
Since you used $$z=8$$ as your Plug-In, the correct answer will match your **goal value** for $$z=8$$. That is not the case here.
>$$2z = 2(8) = \color{red}{16}$$.
Eliminate.

Incorrect.
[[snippet]]
Your **goal value** is what the question asks for:
>$$x-y = 4-2 = \color{blue}{2}$$.
If you try $$z=8$$ in this answer choice, it will not match your goal value.
>$$4z = 4(8) = \color{red}{32}$$.
Eliminate.

$$ \frac{z}{4}$$

$$ \frac{z}{2}$$

$$z$$

$$2z$$

$$4z$$