I dont understand what happened between the 2nd and 3rd step where the multiplication becomes addition

Example 5: Find y' if sin(x + y) = y2 cos x. ( sin( xty ) ) = 2x ( y cosx ) cos ( x ty ) . ( 1 + y' ) = yz(- sinx)+ 2x9. cosx Cos ( x ty ) + y' cos ( x + y ) = - yz sinx + 2 y y' cosx y cos ( xty) - Lyy'cosx = - y sinx - cos (xty) y' ( cos (xty) - 2 4 cosx ) = - y- sinx - cos ( xty) y' = - yz sing - cos(xtx ) Cos( xty) - 2 ycosx