Hey there! The answer to your question is simple. First off, it does not ask for oxygen atoms, so that is just there to mess with you. It says carbon, to hydrogen. That means put the number of carbon, 2, before the number of hydrogen, 4. So, The answer is A - 2:4.
The simplified expression of
is 
<h3>Complete question</h3>
Simplify the expression: (x³ + (-y)³) / (x - y)
Do not include parentheses in your answer.
<h3>How to simplify the expression?</h3>
The expression is given as:

Open the inner bracket

Apply the difference of two cubes to the numerator

Cancel out the common factors

Hence, the simplified expression of
is 
Read more about expressions at:
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2)
4x-10y=12
Subtract 4x from each side.
-10y=-4x+12
Divide both sides by -10
y=2/5x-6/5
3)
13=1/6y+2x
Subtract 2x from each side.
-2x+13=1/6y
Multiply both sides by 6.
-12x+78=y
Flip it around.
y=-12x+78
Hope this helps!