Simplifying
(ab + xy)(7ab + 9xy)

Multiply (ab + xy) * (7ab + 9xy)
(ab(7ab + 9xy) + xy(7ab + 9xy))
((7ab * ab + 9xy * ab) + xy(7ab + 9xy))

Reorder the terms:
((9abxy + 7a2b2) + xy(7ab + 9xy))
((9abxy + 7a2b2) + xy(7ab + 9xy))
(9abxy + 7a2b2 + (7ab * xy + 9xy * xy))
(9abxy + 7a2b2 + (7abxy + 9x2y2))

Reorder the terms:
(9abxy + 7abxy + 7a2b2 + 9x2y2)

Combine like terms: 9abxy + 7abxy = 16abxy
(16abxy + 7a2b2 + 9x2y2)