[(x+y)+(x-y)][(x+y)-(x-y)] = 4xy
[x+y+x-y][x+y-x+y] === negative times positive x equals negative x and negative y times negative equals positive y , don't mind the 4xy on the opposite side!
[2 x][2 y] = 4xy
2x · 2y = 4xy
2 · xy = 4xy
4xy = 4xy
√ = √ They are equal, so this equation and the answer is equal to 4xy.