The answer is below

Step-by-step explanation:

From the diagram attached, to find the length of side AB, we need to use Pythagoras theorem on the right triangles. We can see that AB is the base of the two right triangle.

For the first right triangle with hypotenuse of 13 and height of 12, let x be the base, therefore using hypotenuse:

13² = 12² + x²

169 = 144 + x²

x² = 169 - 144

x² = 25

x = √25 = 5

For the second right triangle with hypotenuse of 15 and height of 12, let y be the base, therefore using hypotenuse:

15² = 12² + y²

225 = 144 + y²

y² = 225 - 144

y² = 81

y = √81 = 9

The length of AB = x + y = 9 + 5 = 14 unit

Since for a square all the sides are equal, therefore the length of each side of the square ABCD is 14 units.

In triangle ADC, the hypotenuse = AC, AD = DC = 14 unit. Using Pythagoras:

AC² = AD² + DC²

AC² = 14² + 14²

AC² = 196 + 196 = 392

AC = √392 = 19.8

The length of its diagonal is about 19.8 unit