$y$ is inversely proportional to $x$. When $y=7, x=9$ a) Work out an equation connecting $y$ and $x$. b) Work out the value of $y$ when $x=21$.

Step 1 ：Since $y$ is inversely proportional to $x$, we have $y = \frac{k}{x}$ for some constant $k$.

Step 2 ：Using the given values $y=7$ and $x=9$, we can find the value of $k$: $7 = \frac{k}{9} \Rightarrow k = 63$.

Step 3 ：The equation connecting $y$ and $x$ is $y = \frac{63}{x}$.

Step 4 ：To find the value of $y$ when $x=21$, we substitute $x=21$ into the equation: $y = \frac{63}{21}$.

Step 5 ：\(\boxed{y=3}\)