Simplifying
2p3x + p2x + -28px

Reorder the terms:
-28px + p2x + 2p3x

Factor out the Greatest Common Factor (GCF), 'px'.
px(-28 + p + 2p2)

Factor a trinomial.
px((-4 + -1p)(7 + -2p))

Final result:
px(-4 + -1p)(7 + -2p)