The value of that expression depends on the value of "x". You can't really simplify it, except that you can factor it by separating the common factor "x".
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x2 - 4x - 21 = (x-7)(x+3)
7
4x + 20
x(4x-2)
It is a quadratic expression
x2-4x-165 = (x-15)(x+11) when factored
4(x - 1)(x + 1)
x^2-4x/x^2-6x+8 = x(x-4)/(x-2)(x-4) = x/(x-2)
x - 2
4x2+4x-8 4(x2+x-2) and (4x+8)(x-1)
Find factors of -5 that can be added to make -4, in this case -5 and 1, so: (x - 5)(x + 1) = x2 - 4x - 5
7x2 -4x -5x2 +2x = 2x2 -2x