What does Interquartile range mean

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?

The table represents a linear function. what is the slope of the function? –6 –4 4 6

When y’all answer can y’all explain too !