Nitrous acid, HNO₂, has Ka = 4.5 × 10⁻⁴…

Step 1. Shortcut. $\sqrt{4.5 \times 10^{-4} \times 0.020} = \sqrt{9.0 \times 10^{-6}} = 3.0 \times 10^{-3}\ \text{mol/L}$, that is $0.003\ \text{mol/L}$.

Step 2. Check. $\alpha = \dfrac{3.0 \times 10^{-3}}{0.020} = 0.15$, that is $15\%$. That is far above 5 per cent, so the shortcut is not safe.

Step 3. Quadratic. Inside the root: $(4.5 \times 10^{-4})^2 + 4 \times 4.5 \times 10^{-4} \times 0.020 = 3.62 \times 10^{-5}$. Its square root is $6.02 \times 10^{-3}$.

Then $x = \dfrac{-4.5 \times 10^{-4} + 6.02 \times 10^{-3}}{2} = 2.78 \times 10^{-3}\ \text{mol/L}$, that is $0.00278\ \text{mol/L}$.

Step 4. pH. $-\log_{10}(2.78 \times 10^{-3}) = 2.56$.