288. $2 \sqrt{8}-7 \sqrt{18}+5 \sqrt{72}-\sqrt{50}$.
Solution. Transform the roots: $2 \sqrt{8}=2 \sqrt{4 \cdot 2}=4 \sqrt{2} ; \quad 7 \sqrt{18}=$ $=7 \sqrt{9 \cdot 2}=7 \cdot 3 \sqrt{2}=21 \sqrt{2} ; \quad 5 \sqrt{72}=5 \sqrt{36 \cdot 2}=5 \cdot 6 \sqrt{2}=30 \sqrt{2} ; \quad \sqrt{50}=$ $=\sqrt{25 \cdot 2}=5 \sqrt{2}$.

Substitute the obtained expressions and combine like terms:

$$
2 \sqrt{8}-7 \sqrt{18}+5 \sqrt{72}-\sqrt{50}=4 \sqrt{2}-21 \sqrt{2}+30 \sqrt{2}-5 \sqrt{2}=8 \sqrt{2}
$$