Answer:
We know that the area of a square of sidelength L, is:
Area = L^2
In this case, we have:
L = 2cm + x cm
And we know that the area of the square is 7cm, then:
Area = 7cm^2 = (2cm + x cm)^2
7cm^2 = 4cm^2 + 4cm*xcm + (x cm)^2
7cm^2 = 4cm^2 + (4*x)cm^2 + x^2 cm^2
Now we can remove all the "cm^2" and just get the numbers:
7 = 4 + 4*x + x^2
We can subtract 4 in both sides to get
7 - 4 = 4 + 4*x + x^2 - 4
3 = 4*x + x^2
This is what whe wanted to prove.