x pangkat 2 -y pangkat 2=(x-y) (x-y)

x²-y² ≠ (x-y)(x-y)
tapi
x²-y² = (x-y)(x+y)

pembuktian
(x-y)(x+y)
= x(x) + x(y) - y(x) - y(y)
= x² + xy - xy - y²
= x² - y²