$\text{a)}\sqrt{25}-\sqrt{9}-\sqrt{25-9}=5-3-\sqrt{16}=5-3-4=2-4=-2$  

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$\text{b)}\sqrt{225-144}+\sqrt{225}-\sqrt{144}=\sqrt{81}+15-12=9+15-12=24-12=12$  

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$\text{c)}\sqrt{324}+\sqrt{576}-\sqrt{324+576}=18+24-\sqrt{900}=18+24-30=42-30=12$  

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$\text{d)}\sqrt{144+256}-\sqrt{144}-\sqrt{256}=\sqrt{400}-12-16=20-12-16=8-16=-8$