"find the segment length "

One expression below in undefined, and the other expression has a well defined value. which expression is undefined and explain why it has no value. which expression is defined? what is the value of that expression and how do you know? what is a different expression using a trig inverse function that is also undefined?

Terri makes a quilt using three sizes of fabric squares the side lenght of each fabric square is the square root of the area

What expression is equivalent to 42x-56y