What is the Measure of arc XY?

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?

In angle pqr find the measure of

Prove that the value of the expression 7^8–7^7+7^6 is divisible by 43.