Answer: x+12
We know that the area is length * width. Since length is x+4, we know
(x+4) * something = x^2+16x+48.
To find that something, note that 48 (from the right side) = 4 (from the left side) * 12. Why is this important? Well, then we know that the something is x+12.
This is because (x+4) * (x+12) = x^2 + 16x + 48. But just focus on this part: 48. Even if we put like 3x+12 or something, that would work out to 3x^2+28x+48. It's 48 every time.
Hope that helped,
-sirswagger21
