So this is saying that there are 8 rows, with 48 tiles in each row. Each row has 6 white tiles, and the rest are purple. That means each row has 42 purple tiles (48-6 = 42).
So you know that each row has 42 purple tiles, and there are 8 rows of tiles, so there are 42 x 8 purple tiles.
42 x 8 = 336
