Explain mathematically how you know 15y=18 and 5y=6 are equivalent

What is the value of x in the diagram below?

Determine whether the quadrilateral below is a parallelogram. justify/explain your answer (this means back it up! give specific information that supports your decision. writing just "yes" or "no" will result in no credit.)

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?