Step 1: Divide the first equation by \(yz\).
Given,
\[
y^2+z^2=ayz.
\]
Dividing by \(yz\), we get
\[
\frac{y}{z}+\frac{z}{y}=a.
\]
Step 2: Divide the third equation by \(xy\).
Given,
\[
x^2+y^2=cxy.
\]
Dividing by \(xy\), we get
\[
\frac{x}{y}+\frac{y}{x}=c.
\]
Step 3: Divide the second equation by \(xz\).
Given,
\[
z^2+x^2=bxz.
\]
Dividing by \(xz\), we get
\[
\frac{z}{x}+\frac{x}{z}=b.
\]
Step 4: Multiply the expressions for \(a\) and \(c\).
Now,
\[
ac=
\left(\frac{y}{z}+\frac{z}{y}\right)
\left(\frac{x}{y}+\frac{y}{x}\right).
\]
Expanding,
\[
ac=
\frac{x}{z}+\frac{y^2}{xz}+\frac{xz}{y^2}+\frac{z}{x}.
\]
Rearranging,
\[
ac=
\left(\frac{x}{z}+\frac{z}{x}\right)
+
\left(\frac{xz}{y^2}+\frac{y^2}{xz}\right).
\]
Since
\[
\frac{x}{z}+\frac{z}{x}=b,
\]
we get
\[
ac=b+\left(\frac{xz}{y^2}+\frac{y^2}{xz}\right).
\]
Therefore,
\[
\frac{xz}{y^2}+\frac{y^2}{xz}=ac-b.
\]
Step 5: Final conclusion.
Hence,
\[
\boxed{ac-b}
\]