Write (x+ y) ^4 in expanded form.
Just check the 4th row of Pascal's triangle:
1 4 6 4 1
attach the descending powers of x and ascending powers of y, so that the powers always add up to 4:
x^4 + 4x^3y + 6x^2y^2 + 4xy^3 + y^4
1 4 6 4 1
attach the descending powers of x and ascending powers of y, so that the powers always add up to 4:
x^4 + 4x^3y + 6x^2y^2 + 4xy^3 + y^4