#### Answer

$m=18$

#### Work Step by Step

1. First we need to find the ratio by setting up two similar sides of the triangles. In this case, we know both hypotenuses, so we use them to set our ratio. It is better to reduce the ratio, if it is possible.
$\frac{14}{21}=\frac{2}{3}$
2. Make following equivalent ratio
$\frac{2}{3} =\frac{12}{m} $
3. We will solve the equation to find m
$\frac{2}{3} =\frac{12}{m}$
$2m=36$
$m=18$