The herfindahl-hirschman index is calculated by summing the square of each company's market share.
In this case, the two firms separately would account for 628 of the herfindahl-hirschman index (12*12+22*22)
If the firms merge, they would account for 1,156 of the herfindahl-hirschman index (34*34)
This is an increase of 528 meaning the market is more concentrated.