Factoring x, we get:
x(y+1)-y-1
But notice that -y-1 can also be -(y+1). Therefore:
x(y+1)-(y+1)
Using techniques from factoring, this can be converted to:
(x-1)(y+1)
I got the x-1 from the factors outside the quantity (y+1). The equation can also be seen as x(y+1)-1(y+1).
