Here's what I get.
For convenient reference, let's list the relative masses of these species.
#M_text(r):14.01color(white)(m)17.03color(white)(m)46.01#
#color(white)(mmm)"N"color(white)(mml)"NH"_3color(white)(ml)"NO"_2#
Ammonia
(a) mg #"N"# to mg #"NH"_3#
#"9 mg N" × "17.03 mg NH"_3/(14.01 "mg N") = "11 mg NH"_3#
#"9 mg N/L" = "11 mg NH"_3"/L"#
(b) mg #"N"# to mg #"NO"_2#
#"9 mg N" × "46.01 mg NO"_2/(14.01 "mg N") = "30 mg NO"_2#
#"9 mg N/L" = "30 mg NO"_2"/L"#
Nitrate
(a) mg #"N"# to mg #"NH"_3#
#"0.5 mg N" × "17.03 mg NH"_3/(14.01 "mg N") = "0.6 mg NH"_3#
#"0.5 mg N/L" = "0.6 mg NH"_3"/L"#
(b) mg #"N"# to mg #"NO"_2#
#"0.5 mg N" × "46.01 mg NO"_2/(14.01 "mg N") = "1.6 mg NO"_2#
#"0.5 mg N/L" = "1.6 mg NO"_2"/L"#
