Maths problem: how to do （b),  and (c). Thanks?

(a)
x = 2V/(zl)-y

(b)
l = 9cm, V = 459cm^3.
z = 6cm, y = x + 3cm.

∴ 459 = (2x+3)×6×9/2
∴ x = 7cm

(c)
V = 459cm^3, l = 18cm, z = 3cm
∴ x + y = 2V/(zl)
      = 2(459)/(18×3) = 17cm ≠ 18cm