Well if the perimeter is 28 inches, each side of the square is 7 inches. 28/4 = 7 because all sides of a square are the same.
If we split the square in half to create 2 triangles we can find the length of the diagonal. The two sides are the bases of 7 inches.
Use the Pythagorean theorem to solve for the hypotenuse or the diagonal.
7^2 + 7^2 = diagonal ^ 2
Therefore, the diagonal equals 9.9 inches.