The answer are 12 4 x x+4 -16 16 x+12 x^2

Step 2: {4, x+4}
Factoring x²+4x, you get x(x+4). If you call the length x, then the width is the other factor, x+4.

Step 4: {16}
You know this two ways. -192/-12 = 16, and 4 -(-12) = 16, where 4 is the coefficient of x in step 3.

Last line: {-16, 16}
The values of x that make the factors zero are -16 and 12. Since these are supposed to represent lengths, an appropriate answer cannot be negative (-16). The width is x+4 = 12+4 = 16.