If there are x lines in the front area, there are (x+8) lines in the back area.

Number of seats in the last row of the front row: 20+2(x- 1)=2x+ 18.

Front seat: [20+(2x+18)] * x÷ 2 = x (x+19) = x2+19x.

Rear seat: (2x+18) * (x+8) = 2x2+34x+144.

So: 2 (x2+19x)+108 = 2x2+34x+144.

2x^2+38x+ 108 = 2x^2+34x+ 144

4x=36

x=9

So the number of seats in the front area: x2+19x = 9 2+19 * 9 = 252.

Seats in the rear area: 2x2+34x+144 = 2 * 92+34 * 9+144 = 612.

So the total number of seats: 252+6 12=864.