#### Answer

$R= \frac{27.5}{\sqrt{x}}$

#### Work Step by Step

Since $R$ is inversely proportional to the square root of $x$, we have:
$R= \frac{k}{\sqrt{x}}$
We are given: $R=2.5$ and $x=121$, so we have:
$2.5= \frac{k}{\sqrt{121}}=\frac{k}{11}$
$k=2.5*11=27.5$
So the equation is:
$R= \frac{27.5}{\sqrt{x}}$