What core clocks ( mhz ) have a 21.3 memory bandwidth ( gb/s )?
CREATE TABLE "table1_25839957_5" (
  "graphics" text,
  "launch" real,
  "market" text,
  "cpu" text,
  "code_name" text,
  "device_id" real,
  "core_clock_m_hz" text,
  "execution_units" real,
  "shader_model" text,
  "direct_x" text,
  "open_gl" text,
  "open_cl" text,
  "memory_bandwidth_gb_s" text,
  "dvmt_mb" real,
  "cvt_hd" text,
  "qsv" text
);
SELECT "core_clock_m_hz" FROM "table1_25839957_5" WHERE "memory_bandwidth_gb_s"='21.3';