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.