Please explain why the square root of 4x^2 + 48x + 144 = 2x + 12?

(2x + 12)^2
  = (2x + 12)(2x + 12)
  = 2x(2x + 12) + 12(2x + 12)
  = 4x^2 + 24x  +  24x + 144
  = 4x^2 + 48x + 144
So we have
  (2x + 12)^2 = 4x^2 + 48x + 144

Taking the square root of both sides, we get
  2x + 12 = √(4x^2 + 48x + 144)