Let \( R \) be the focus of the parabola \( y^2 = 20x \) and the line \( y = mx + c \) intersect the parabola at two points \( P \) and \( Q \). Let the point \( G(10, 10) \) be the centroid of the triangle \( PQR \). If \( c - m = 6 \), then \( (PQ)^2 \) is:
