“Expressing it in terms of Y” simply means to get Y by itself. It will probably be easiest to see if you begin by simplifying the denominator (the denominator is the bottom portion of a fraction).
Your denominator is: 500 * (500 / Y)
That’s the same thing as: (500 * 500) / Y = 250,000/Y
Now you have:
X = Y / (250,000 / Y)
Of course, you know how to divide fractions, so you recognize that this is the same thing as:
X = Y * (Y / 250,000)
Can you see what to do from here? (Even if not, you need to try to figure it out, and we will talk through your thought process. I’m not going to simply give you the answer.)