jmc

First, we combine like terms in the expression: $$\begin{array}{rl}(9x^5& +25x^3-4)-(x^5-3x^3-4)\\ & =9x^5+25x^3-4-x^5+3x^3+4\\ & =8x^5+28x^3.\end{array}$$ We can factor out a $4x^3$ from the expression, to get $$8x^5+28x^3=\boxed{4x^3(2x^2+7)}.$$