Answer:
When raising an expression of that format to any power, after grouping like terms, you'll always find that the odd numbered terms are positive, and the even numbered terms are negative.
For example:
(x - y)² = x² − 2xy + y²
(x - y)³ = x³ − 3x²y + 3xy² − y³
(x - y)⁴ =x⁴ − 4x³y + 6x²y² − 4xy³ + y⁴
etc.
