Because you're multiplying [tex]2x[/tex] by [tex]4x[/tex]. [tex]2*4[/tex] is [tex]8[/tex], [tex]x*x[/tex] is [tex]x^2[tex], and [tex]8*x^2[/tex] is [tex]8^2[/tex]. Since you're multiplying them all together, it doesn't matter what order you multiply them in. For example, [tex]2*x=2x[/tex], [tex]x*4=4x[/tex], and [2x*4x=8x^2[/tex] (but that's just the same.)
Hope that helps! :)
