Help please

You mean$\dfrac{1}{2}\times \dfrac{4}{3}\times \pi \times 253-\dfrac{2}{3}\times \pi \times 253$?

$\dfrac{1}{2}\times \dfrac{4}{3}\times \pi \times 253-\dfrac{2}{3}\times \pi \times 253\\ = \dfrac{2\times 253\times \pi}{3}-\dfrac{2\times 253\times \pi}{3}\\ = 0$

Or if you really mean 1/2 * 4/3 * pi * 253 = 2/3 * pi * 253, that is an identity.