Buffer solutions are used by biological mammalian systems to maintain the $\mathrm{pH}$ of blood plasma within a narrow range. In these systems, the compound from which this solution is obtained is $\ce{CO2}$, produced in cell respiration, which is converted into $\ce{HCO3-}$ and $\ce{H2CO3}$ inside the red blood cells. Using the Henderson-Hasselbalch equation, answer: $\mathrm{p}K_\mathrm{a} = 6.35$. What is the ratio between $\ce{HCO3-}$ and $\ce{H2CO3}$ in a blood sample with $\mathrm{pH} = 7.4$?
The problem is I try to make the exercise in this way:
$$\mathrm{pH}= \mathrm{p}K_\mathrm{a} + \log \left(\frac{\ce{[HCO3-]}}{\ce{[H2CO3]}} \right)$$
BUT in the solution they make $$\mathrm{pH} =\mathrm{p}K_\mathrm{a} - \log \dfrac{[\ce{HCO3-}]}{[\ce{H2CO3}]}$$
They make the inverse of $$\mathrm{pH} = \mathrm{p}K_\mathrm{a} + \log \dfrac{[\ce{H2CO3}]}{[\ce{HCO3-}]}$$
And this don't make sense for me. I don't understand.