The equations of the sides \( AB \), \( BC \) and \( CA \) of a triangle \( ABC \) are: \( 2x + y = 0 \), \( x + py = 21a \) \( (a \neq 0) \) and \( x - y = 3 \) respectively. Let \( P(2, a) \) be the centroid of \( \triangle ABC \). The \( (BC)^2 \) is equal to:
