#### Answer

$m=60$

#### Work Step by Step

$m$ varies jointly as $x$ and $y$ therefore $m=kxy$
$m=10$ when $x=2$ and $y=14$. Substitute these values into $m=kxy$ to obtain:
$m=kxy
\\10=k(2)(14)
\\10=28k
\\\frac{10}{28}=\frac{28k}{28}
\\\frac{5}{14}=k$
Thus, the equation for $m$ is $m=\frac{5}{14}xy$.
the values of $x$ and
To find the value of $m$ when $x=21$ and $y=8$, substitute 21 to x and 8 to y in the equation above to obtain:
$\require{cancel}
\\m=\frac{5}{14}xy
\\m=\frac{5}{14}(21)(8)
\\m=\frac{5}{\cancel{14}2}(\cancel{21}3)(8)
\\m=\frac{5}{2}(3)(8)
\\m=\frac{5}{\cancel{2}}(3)(\cancel{8}4)
\\m=60$