how would u factor this...?

Since you find x in the x^2 and the 4x, therefore x is the common factor. So all what you have to do is to take this common factor which is the x from both, the x^2 and the 4x, and leave what remains in side parenthesis. So you will have x(x - 4)

x ( x - 4 )

x(x-4)

x(x-4)

x^2 - 4x
= x(x - 4)

x(x-4)

you could also do

x((x^.5-2)(x^.5+2))

(x-2)(x+2)