hey sorry for the sloppy graph i attached. I don't know how to really use the graph software. And um, please don't look at the numbers on the x-axis and y-axis. I don't know how to erase them. There's number on the graph given.

The figure shows the graph of y=x^2 and y=a-x^2 for constant a. If line segment PQ=6. What does the constant "a" equal? I don't get why the answer is 18.

The attachment might seem a little small, the graph of the with concave down (opening down?) is the equation y=a-x^2 and the graph with concave graph is y=x^2. The line segment between the two graphs (between the two points they intersect) is PQ which equals 16.