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To simplify the given expression, we first need to factor the expressions in the numerators and denominators of the fractions:

1. frac{x^2-25}{x^2+6x+9} = frac{(x+5)(x-5)}{(x+3)^2}

2. frac{x-5}{x+3}

3. frac{6x}{3x+9} = frac{6x}{3(x+3)}

Now, we can rewrite the expression with the factored forms:

frac{(x+5)(x-5)}{(x+3)^2}div frac{x-5}{x+3}-frac{6x}{3(x+3)}

Next, we can simplify the division by multiplying the first fraction by the reciprocal of the second fraction:

frac{(x+5)(x-5)}{(x+3)^2} * frac{x+3}{x-5} - frac{6x}{3(x+3)}

Now, we can combine the fractions by finding a common denominator:

frac{(x+5)(x-5)(x+3)}{(x+3)^2(x-5)} - frac{6x(x-5)}{3(x+3)(x-5)}

Simplify the numerators:

frac{(x^2-25)(x+3)}{(x+3)^2(x-5)} - frac{6x^2-30x}{3(x+3)(x-5)}

Now, we can combine the fractions:

frac{(x^2-25)(x+3) - 2(2x^2-10x)}{3(x+3)(x-5)}

Simplify the numerator:

frac{x^3 - 25x + 3x^2 - 75 - 2x^2 + 10x}{3(x+3)(x-5)}

Combine like terms:

frac{x^3 + x^2 - 15x - 75}{3(x+3)(x-5)}

Therefore, the simplified expression is frac{x^3 + x^2 - 15x - 75}{3(x+3)(x-5)}.