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First, expand to make it easier
y=x(x^2+5)(x^2+5)
y=x(x^4+10x^2+25)
y=x^5+10x^3+25x
differentiation
y'=5x^4+30x^2+25 is the derivitive
The dance would /cost/ the school 50 dollars, making the profit -50. The dance tickets make 300, and the DJ takes away 250, making a 50 profit. However, the decorations cost 100, taking away that from 50, making -50.