How did they get the value of pKa in this example?

You consult a list of pKa or Ka values:
My reference list gives for HCOOH:
Formic acid HCOOH: Ka = 1.77 x 10^-4
pKa = -log Ka = -log (1.77*10^-4) = 3.75
These values vary slightly depending on the source.

Many textbooks and webpages give pKa values.

For example, refer to:
http://clas.sa.ucsb.edu/staff/Resource%20folder/Chem109ABC/Acid,%20Base%20Strength/Table%20of%20Acids%20w%20Kas%20and%20pKas.pdf
pKa for HCOOH = 3.75
The value is close to 3.77.

pKa values from different sources may be slightly different.