CREATE TABLE table_name_56 (erp_w INT, class VARCHAR, frequency_mhz VARCHAR)

Name the sum of ERP W for class of d and frequency mhz of 98.7
SELECT SUM(erp_w) FROM table_name_56 WHERE class = "d" AND frequency_mhz = 98.7
SELECT SUM("erp_w") FROM "table_name_56" WHERE "class" = "d" AND "frequency_mhz" = 98.7