1/4u^2 - 9/4

Since you didn't bother to read the rules before trying to get someone else to do your homework... here they are.

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First off, do you know how to factor the difference of two squares when fractions AREN'T involved?

If you can do that, then just remember that when you square a fraction, you simply square the numerator and denominator individually. Hence a fraction like 3/4 becomes 9/16, or 2/5 becomes 4/25. So when you need to work backwards, starting with a square and needing to find the square root, just do the opposite. When you take the square root of a fraction like 9/4 you can just take the square root of the numerator and denominator separately (i.e. 9/4 becomes 3/2).

If that doesn't help, or you're still unsure how to proceed, post back and let us know which part or parts are confusing you. We'll help you out.