joint and combined variation

[ibueno1]

i really need help on this problem you are supposed to solve

a varies jointly as b and c and inversely as d.one set of values is a=0.525, b=6,c=7, and d=8.find a when b=12,c=7 and d=3

i would really appreciate your help :D

[Gene]

The general formula is
a=k(bc/d)
Use the given set of values to find k then plug in the "question" values.