a^x=b^y=c^z=k(say)

then,

a=k^(1/x),b=k^(1/y),c=k^(1/z);..........(1)

given b^2=ac

{K^(1/y)}^2=k^(1/x)*k^(1/z);

k^(2/y)=k^(1/x+1/z)

2/y=1/x+1/z

2/y=x+z/xz

hence the answer is 2/y "100% it is correct answer"