Maths problem:how to do (b), thanks?

(bi)

一條鐵線的長度為x, 另一條鐵線的長度則為(36-x)
(x ÷ 4)² + ((36 - x) ÷ 4)² = A
(x² ÷ 16) + ((1296 - 72x + x²) ÷ 16) = A
(1296 - 72x + 2x²) ÷ 16 = A
A = 1/8x² - 9/2x + 81

(bii)

8A ≥ 324
8 × (1/8x² - 9/2x + 81) ≥ 324
x² - 36x + 648 ≥ 324
x² - 36x + 324 ≥ 0

x = (-(-36) ± √((-36)² - 4 × 324)) ÷ 2
x = 18

x² - 36x + 324
= (18)² - 36(18) + 324
= 0

∵ 8A ≥ 324成立
∴ 曉琳剪出的兩塊布料的總成本不會少於$324