Brockton will receive an $11 million state grant to help Boston-based Trinity Financial with a $100 million redevelopment project in the downtown area. It’s the second installment of a total of $15 million from the MassWorks Infrastructure Grant Program to aid the company in a two-phase plan to redevelop the Enterprise Block, at the junction of Main, Montello, and Centre streets and Petronelli Way. The project is the most significant public-private investment in downtown Brockton in more than 20 years, said Mayor Linda Balzotti. The plan combines a mix of housing and artists’ and office space in the converted 55,000-square-foot Enterprise Building, as well as a new building that replaces the Gardner Building. The Gardner, a former factory, was supposed to be part of the renovation plan but had to be razed this year because of its condition.