How do i solve the system of equations using substitution

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?

Which expression will have the same sum as 3_8 + 1/4?

Don't know how to find x given these segment lengths