Answer:
To find the diagonal length, you can use the Pythagorean theorem.
If one side is 28 inches, and the board is a square, the other side must also be 28 inches.
a = side 1
b = side 2
c = diagonal length
a^2 + b^2 = c^2
(28)^2 + (28)^2 = c^2
784 + 784 = c^2
1568 = c^2
Square root both sides.
c = about 39.6 inches diagonal length