Each row grows by 2 musicians, and there are 8 rows total. You can find the total expansion of musicians by multiplying these together.
(2*8) = 16
The first row has 5 musicians, so we’ll use this as our base number. Each row adds musicians until we reach the final row, so we’ll add the base number to the number we got before to reach the total musicians in the final row
16 + 5 = 21
21 musicians in the final row