What is the answer to x+y=3

x=-y+3

(x+y) ² = (x+y) (x+y) Then you FOIL (First, outer, inner, last)

(x+y) ² = (x+y) (x+y) = xx + xy + xy + yy [and when you combine like terms] = x² + 2xy + y²

(x+y) 3 = (x² + 2xy + y²) (x+y) Then you FOIL (First, outer, inner, last)

(x+y) 3 = (x² + 2xy + y²) (x+y) = x2x + 2xxy + xy2 + x2y + 2xyy + y2y [and when you combine like terms] = x3 + 3x2y + 3xy2 + y3