Answer
$P(rainy~|~cloudy)=\frac{0.21}{0.37}\approx0.568$
Work Step by Step
$P(cloudy)=37$% $=0.37$
$P(cloudy~and~rainy)=21$% $=0.21$
Using the Conditional Probability Rule (see page 288):
$P(rainy~|~cloudy)=\frac{P(cloudy~and~rainy)}{P(cloudy)}=\frac{0.21}{0.37}\approx0.568$
