That is the Identity Property of Addition.
If it is adding and the whole number stays the same it is Identity Property of Addition.
If it is multiplying then it would be the Identity Property of Multipulcation. For example for this problem it would be 6x1 in multipulcation in which the whole number would stay the same.
Here are descriptions of all the properties:
https://wikis.engrade.com/mathproperties1
We can use the 9 inches wide to determine that 18 inches of the 42 inches are the wide sides. 42 - 18 = 24, so we divide 24 by how many long sides there are, 2. 24 ÷ 2 = 12. The box was 12 inches long.
Area=legnth times width=31
the frame must go around the frame with equal legnth, x
so
the total area (meaning the frame +picture) is 5+x by 6+x
31=(5+x)(6+x)
expand
31=30+11x+x^2
minus 31 both sides
0=-1+11x+x^2
x^2+11x-1=0
answer is third one
