A certain restaurant has exactly 19 tables. Some of these tables are larger tables that can seat 6 people and the others are smaller tables that can seat just 4 people. If a total of 90 people can be seated at this restaurant, how many of the tables must be of the larger kind??

number of large tables ---- x

number of smaller tables = 19-x

6x + 4(19-x) = 90

solve for x