How would you differentiate this problem using the quotient rule?

Rewrite the square root of X as X^ 1/2. Then take the derivative. Since 1/3 is a constant with respect to X you can pull that out. So, you should have
1/3 d/dx [ (2x^2-x^5)/3x^1/2].

Then factor x^2 out of 2x^2-x^5.

Then by using the negative exponent rule you should have

1/3 d/dx [ x^2(2-x^3)x^-1/2 ]

Then multiply x^2 by x ^-1/2 by adding the exponents.

1/3 d/dx [ x^3/2 (2-x^3)]

Then differentiate.

You should get x^1/2 - (3x^7/2)/2.

I hope that helps.

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