Sorry, very simple question i'm stuck on.

Example in my book shows:

$\displaystyle 5^\frac{1}{2}-5^\frac{3}{2}+5^\frac{5}{2} = \sqrt{5} - 5\sqrt{5} + 5^2\sqrt{5} = (1 - 5 + 25)\sqrt{5} = 21\sqrt{5} $

My Q.

$\displaystyle 7^\frac{1}{2}-7^\frac{3}{2}+7^\frac{5}{2} $ So I just substituted to get $\displaystyle \sqrt{7} - 7\sqrt{7} + 7^2\sqrt{7} = (1 - 7 + 49)\sqrt{7} = 43\sqrt{7} $

Which is way off the answer in my book of $\displaystyle 4\sqrt[3]{7}$

thanks!