$$ \begin{aligned} 3(2x+1)(-x+3)-(2x+5)^2 &= -(-(-3(x+5))+10x^2)&& \text{simplify left and right hand side} \\[1 em]3(2x+1)(-x+3)-(4x^2+20x+25) &= -(-(-(3x+15))+10x^2)&& \\[1 em](6x+3)(-x+3)-(4x^2+20x+25) &= -(-(-3x-15)+10x^2)&& \\[1 em]-6x^2+18x-3x+9-(4x^2+20x+25) &= -(3x+15+10x^2)&& \\[1 em]-6x^2+15x+9-(4x^2+20x+25) &= -3x-15-10x^2&& \\[1 em]-6x^2+15x+9-4x^2-20x-25 &= -10x^2-3x-15&& \\[1 em]-10x^2-5x-16 &= -10x^2-3x-15&& \text{move all terms to the left hand side } \\[1 em]-10x^2-5x-16+10x^2+3x+15 &= 0&& \text{simplify left side} \\[1 em]-10x^2-5x-16+10x^2+3x+15 &= 0&& \\[1 em]-2x-1 &= 0&& \text{ move the constants to the right } \\[1 em]-2x &= 1&& \text{ divide both sides by $ -2 $ } \\[1 em]x &= -\frac{1}{2}&& \\[1 em] \end{aligned} $$

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