Question 209412
y^2(a+b) -4(a+b)
.
This  factor  is  hard  to  visualize,  If  we  had  lm - nm,,,,m(l-n) is  ok,,,????
.
in  this  case,,,  m=(a+b),,,l=y^2,,,n=4
.
or,,,(a+b)(y^2-4),,,review  several  times  until  confident
.
unfortunately  the  result  still  has  another  factor,,y^2-4,,  which is  difference  of  squares
.
(a+b)(y+2)(y-2),,,,answer
.
.
to  check,,,multiply  factors
.
(a+b)(y+2)(y-2)
.
(a+b) (y^2 -2y+2y-4)
.
(a+b) (y^2 -4)
.
ay^2 -4a +by^2 -4b

ay^2 +by^2 -4a -4b

y^2(a+b) -4(a+b),,,,,,,,ok
.