Table: ebook_readers
Columns: maker
model
intro_year
screen_size_inch
self_lit
screen_type
weight
screen_pixels
touch_screen
micro_sdhc

Question: For Amazon.com's device with a weight of 221g (7.8oz) how many screen pixels does the device have?
SQL: SELECT "screen_pixels" FROM ebook_readers WHERE "maker" = 'amazon.com' AND "weight" = '221g (7.8oz)'