For △ P QR, shown here, points Q ( − 2/3 , q ) and R ( 2/3 , r ) lie on the graph of y = ax^2 , and point P ( − 2/3 , p ) lies on the graph of y = − ax^2 , where a > 0 . If the area of △ P QR is 16...

For △ P QR, shown here, points Q ( − 2/3 , q ) and R ( 2/3 , r ) lie on the graph of y = ax^2 , and point P ( − 2/3 , p ) lies on the graph of y = − ax^2 , where a > 0 . If the area of △ P QR is 16 units^2, what is the value of a ?