First, let's assume that the numbers given corresponds to the length and width of the area.
*area = l x w*
a1 (upper area) = 8 x 2
a2 (lower area) = 6 x 2
a3 (front row) = 4 x ?
It's logical to say the a3 should have 2 as it's width. By this, we can now calculate the total length and width. For these 3 given, we can say that the total length is 18 (8 + 6 + 4) and we can get the percentage per location and we can described later how many seats it represent from the total of 138.
a1 = 8 ÷ 18 X 100
a1 = 44.44%
a2 = 6 ÷ 18 X 100
a2 = 33.33%
a3 = 4 ÷ 18 X 100
a3 = 22.22%
We now know the percentages that each area represents, so we can used these to solve for the number of seats.
a1 = 138 x 0.44
a1 = 61 seats
a2 = 138 x 0.33
a2 = 46 seats
a3 = 138 x 0.22
a3 = 31 seats
To check, we can add 61 seats + 46 seats + 31 seats, and we will get a total of 138 seats.