See: List of castles, Castles in England, Castles in Scotland, Castles in Northern Ireland
Anglesey
Clwyd
Dyfed
Glamorgan
Gwent
Gwynedd
Powys
South Glamorgan
West Glamorgan
... (i.e., in this case, a quantity that does not depend on x) to the expression to the left of "=", that will make it a perfect square trinomial of the form x2 + 2xy + y2. ...