Solving $x^4 - 10x^3 + 21x^2 + 40x - 100 = 0$ Could someone please explain how to solve this:
$$x^4 - 10x^3 + 21x^2 + 40x - 100 = 0$$
not the answer only, but a step-by-step solution.
I tried to solve it, with the help of Khan Academy, but still I have no idea how to correctly solve it.
Thank you so much in advance!

First note that $x= \pm 2$ are the roots of the equation. Hence using the remainder theorem, we can proceed as follows:
$$ x^3(x-2) - 8x^2(x-2) +5x(x-2) + 50(x-2)=0$$ 
$$\qquad \Rightarrow (x-2)(x^3 -8x^2 +5x +50) =0$$
$$ (x-2)( x^2(x+2) -10x(x+2) +25(x+2)) =0 $$
$$\qquad \Rightarrow (x^2-4)(x^2-10x+25)=0 $$
Hence we can solve it as 
$$ (x^2-4)(x-5)^2 =0 $$ or $$x=\{ -2,+2,5 \}$$
