6b^3+48b/b^3+64-3b^2/b^2-4b+16

= (6b^3+48b) / (b + 4) (b²-4b+16) - 3b² / (b²-4b+16) =

= (6b^3+48b - 3b² (b+4)) / (b+4) (b²-4b+16) =

= (6b^3+48b - 3b^3 - 12b²) / (b+4) (b²-4b+16) =

= (3b^3 + 48b - 12b²) / (b+4) (b²-4b+16) =

=3b (b² - 4b+16) / (b+4) (b²-4b+16) =

=3b / (b + 4)