Hello, everyone.
I have multiple regression

scorei = β0 + β1clsizei + β2el_pcti + β3comp_stui + β4avginci + ui,

score - the district average of the math and reading test scores of 5th grade students.
avginci-district average income

How I can calculate the level of average income after which it is estimated to decrease test scores?
Thanks for help in advance)