Simplifying
(x + y)(x + y)

Multiply (x + y) * (x + y)
(x(x + y) + y(x + y))
((x * x + y * x) + y(x + y))

Reorder the terms:
((xy + x2) + y(x + y))
((xy + x2) + y(x + y))
(xy + x2 + (x * y + y * y))
(xy + x2 + (xy + y2))

Reorder the terms:
(xy + xy + x2 + y2)

Combine like terms: xy + xy = 2xy
(2xy + x2 + y2)