<s>[INST] Quadratic formula and factoring are leading to different answers $$x^{ 2 }-2x-15=0$$
By factoring, I get:
$$(x-5)(x+3)$$
Which has the solutions: 
$$x=5, x=-3$$
However when I use the quadratic formula (which is what the book saids to use), I get 
$$\frac { 2 \pm \sqrt { 4-(4\cdot1\cdot(-15)) }  }{ -2 } =$$
$$\frac { 2\pm 8 }{ -2 } $$
Which I evaluate to be $$x=-5, x=3$$
Where am I going wrong?
 [/INST] $$ax^2+bx+c=0\implies x=\frac{-b\pm\sqrt{b^2-4ac}}{2a}$$
$$\implies x^2-2x-15=0\implies x=\frac{2\pm\sqrt{(-2)^2-4\cdot1(-15)}}{2\cdot1}=\frac{2\pm8}2=5,-3$$
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