ocr chemistry acid question

Healthy blood at a pH of 7.40 has a hydrogencarbonate : carbonic acid ratio of 10.5 : 1.
A patient is admitted to hospital. The patient’s blood pH is measured as 7.20.
Calculate the hydrogencarbonate : carbonic acid ratio in the patient’s blood.

http://pmt.physicsandmathstutor.com/download/Chemistry/A-level/Past-Papers/OCR-Old/Unit-5/January%202012%20MS%20-%20Unit%205%20OCR%20Chemistry%20A-level.pdf

q 4dii

I don't underrstand the last part of this question, how did they get the ratio and rearranged it?

This is a buffers question.

ka = [H+][A-]/[HA]

if you are given pH you can work out [H+]
If you are given ka you can work out ratio of [A-] to [HA]

How do you rearrange it at the end?
I got
4.18 x 10^-7 = 6.31 x 10-8 x HCO3- / H2CO3

not sure how to rearrange this...

how do you rearrange it at the end?
I got
4.18 x 10^-7 = 6.31 x 10-8 x hco3- / h2co3

not sure how to rearrange this...