Answer:
His diagonal brace need to be 15.55 feet long in the kitchen.
Explanation:
Given : The wall of the kitchen is in the shape of a square.
Area of the wall = 121 sq ft
Let us assume the each side of wall = a ft
Now, AREA OF THE SQUARE = Side x Side
⇒ a x a = 121 sq units

So, each side of the kitchen wall is 11 ft.
Now,as we know in a square : EACH ANGLE IN THE SQUIRE IS A RIGHT ANGLE.
So, by Pythagoras Theorem, Diagonal D is:

Hence, his diagonal brace need to be 15.55 ft long in the kitchen.