W varies jointly as x & y and inversely as v²
so the expression is
W = k.xy/v² / k = constant
to solve this kind of problem we must find the constant k first
1200 = k( 4 . 9)/6²
k 1200
W = 1200 .(3 . 12 )/9² /just plug in the new values
= 1200 . 36/81
~ 533.3
do you mean P varies directly as s³ & inversely as b
so the expression is
P = k s³/b
3/4 = k 2³/6
3/4 = k 8/6
k = 3/4*6/8= 9/16
6 = (9/16 . 4³)/b
b = ( 9/16 . 4³)/6
b = 6