Akite has a square base with dimensions of 4 feet by 4 feet. what is the length of one of the diagonal crossbars that will be needed to stabilize the kite?

The diagonal crossbar would create a hypotenuse so you can use Pythagorean Theorem of a^2 + b^2 = c^2. The two '4 feet' lengths would be the legs (a and b). 

So now just plug in: 4^2 + 4^2 = c^2. Simplify to 16 + 16 = c^2. Simplify again to 32 = c^2. Square root both sides to isolate the variable c. to get C = sq. 

The length of one of the diagonal crossbars is  feet.