What is the ph of a 0.085 m solution of nitrous acid (hno2) that has a ka of 4.5 × 10-4?

Data Given:
                  Molarity  =  0.085 M

                  Ka  =  4.5 × 10⁻⁴
To Find:
                  pH  =  ?
Solution:
             First find out the concentration of [H⁺] Ions using following formula,

                 [H⁺]  =  

Puting values,

                 [H⁺]  =  

                 [H⁺]  =  

                 [H⁺]  =  0.00618465843
Or,
                 [H⁺]  =  6.18 × 10⁻³

Calculate pH as,

                 pH  =  -log [H⁺]

                 pH  =  -log (6.18 × 10⁻³)

                 pH  =  2.20

How do we find the answer?