`a,`

`(1/(x+y) - 1/(x-y)) . ((x^2)/(4y) - (2x-y)/4)`

`=(x-y-x-y)/((x+y)(x-y)) . (x^2 -y(2x-y))/(4y)`

`= (-2y)/((x+y)(x-y)) . (x^2 -2xy+y^2)/(4y)`

`= ( -2y)/((x+y)(x-y)) . ((x-y)^2)/(4y)`

`= (-(x-y))/(2(x+y))`

`b,`

`(1/(x^2 -8x+16) - 1/(x^2 -16)) : (3 - 12/(x+4))`

`= (1/((x-4)^2) - 1/((x-4)(x+4))) : (3(x+4)-12)/(x+4)`

`= (x+4-x+4)/((x-4)^2 (x+4)) : (3x+12-12)/(x+4)`

`=8/((x+4)(x-4)^2) : (3x)/(x+4)`

`= 8/(3x(x-4)^2)`.