3x(y - 4)(y + 4)
3xy² - 48x ( take out the common factor 3x )
= 3x( y² - 16 )
y² - 16 is a difference of squares, which factors in general as
a² - b² = (a - b)(a + b)
here a = y and b = 4
y² - 16 = (y - 4)(y + 4), hence
3xy² - 48x = 3x(y - 4)(y + 4)
