Maths problem, thanks?

(a) f(x) = x²-36x+648
　　　= x²-(2)18x+18²-18²+648
　　　= (x-18)²+324
∴ vertex of graph = (18,324)

(b) (i) length of the other piece = 36-x cm
sum of areas of 2 squares, A
= (x/4)² + [(36-x)/4]²
= (x²+36²-72x+x²) /16
= (2x²-72x+1296) /16
= (x²-36x+648) / 8 , or
= [(x-18)²+324] / 8　　--- from (a)

(ii) From (i) :
When sum of areas (A) of 2 squares is minimum, x = 18
∴ corresponding sum of areas = 324/8 = 40.5 cm²