The sum of supplementary angles is \(180^{\circ}\). I was hoping you would see that:
[MATH]\angle QR'R=\angle QRR'[/MATH]
since [MATH]\triangle QRR'[/MATH]
is isosceles. And then you would note that \(\angle QR'R\) and \(QR'P\) lie along a straight line. Thus, the two possible angles \(R\) must be supplementary.
Observe also that the altitude \(h\) of the triangle is:
[MATH]h=11\sin(37^{\circ})\approx6.619965254672531[/MATH]
Since this is less than 9, a line segment of length 9 originating at \(Q\) will have two possible points of intersection with the base of the triangle, at points \(R\) and \(R'\).