[DutchGirl13]

I'm given these equations:
HA=C_HA-A^-
H⁺=OH^-+A^-
OH^-=Kw/H⁺
Ka=H⁺*A^-/HA

Using rearrangements of the first three substituted into the last one, I'm supposed to derive 0= H+³+KaH+^2-(Kw+KaC_HA)*H⁺-Kw*Ka.

My final Ka= H+*(H⁺-(Kw/H+))/(C_HA-(H⁺-(Kw/H+))
This leads to KaCHA-KaKw/H⁺-KaH⁺-H⁺^2+Kw=0,not the cubic. Where am I going wrong in my algebra?

[DutchGirl13]

Oops. I apparently forgot to hit "end'' after doing subscripts, or did something else wrongly.

0= H+3+KaH+^2-(Kw+KaCHA)*H+-Kw*Ka.

My final Ka= H+*(H+-(Kw/H+))/(CHA-(H+-(Kw/H+))
This leads to KaCHA-KaKw/H+-KaH+-H+^2+Kw=0, not the cubic. Where am I going wrong in my algebra?

[AWK]

One set of brackets missing
Ka= (H+*(H+-(Kw/H+)))/(CHA-(H+-(Kw/H+))

[Borek]

One set of brackets missing
Ka= (H+*(H+-(Kw/H+)))/(CHA-(H+-(Kw/H+))

It doesn't change anything, x*y/z is not different from (x*y)/x  

That's what happens when you list H+ instead of H⁺ or [H⁺] or [H+] or just H.

[AWK]

My correction of formula slightly improve its correct understanding (assuming "H+" always means concentration of H⁺).

Sorry,
something goes wrong on the forum (may be my fault) and instead of quoting I edited Borek  post.
Moreower, a  multiplication in nominator  (dot) is visible here as short minus sign
should be read as  [H3O+]*([H3O+]-Kw/[H3O+])

[Borek]

Strange, looks like one of my posts disappeared.

My final Ka= H+*(H⁺-(Kw/H+))/(C_HA-(H⁺-(Kw/H+))
This leads to KaCHA-KaKw/H⁺-KaH⁺-H⁺^2+Kw=0,not the cubic. Where am I going wrong in my algebra?

Hard to tell what you did wrong not seeing your algebra.

Ka= H+*(H⁺-(Kw/H+))/(C_HA-(H⁺-(Kw/H+)) looks to me like already 3^rd degree equation.