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

(a) Let u=kv²w³ 
54 = k2²3³
∴ k = 54/108 = 1/2
=>  u=v²w³/2

(b) When v=3, w=2,
 u=3²2³/2 =36

(c) 
A quantity is said to "vary jointly" as 2 quantities, if it equals a constant k times their product. 
Now, u = ½v²w³ = (½vw²)vw,
which means u equals (½vw²) times the product of v & w
However, ½vw² is not a constant.
∴ u does not vary jointly as v & w.