Calculating ratio of bicarbonate ion to carbonic acid in blood
For these types of problems, the general approach would be to use the Henderson-Hasselbalch equation. For a general weak acid/conjugate base system, this has the form: $$\mathrm{pH}=\mathrm pK_\mathrm a+\lg\left(\frac{\ce{A-}}{\ce{HA}}\right)$$ Since you are given $\mathrm{pH}$ and have found $\mathrm pK_\mathrm a$, it won't require much more work to obtain the ratio.