There are 6 rows with 18 chairs in each row. in the center of the chairs, 4 rows of 6 chairs are brown. the rest of the hairs are blue. how many chairs re blue?

6 rows with 18 chairs each.

Total number of chairs = 6 * 18 =  108 chairs.

4 rows of 6 chairs are brown : 

Total = 4 * 6 = 24 chairs are brown.

Number of chairs that are blue =  Total - number of brown = 108 - 24 = 84

84 chairs are blue.

I hope this explains it.