Use the Chain Rule to find $\partial z/\partial s$ and $\partial z/\partial t$.
$z =\tan\dfrac{u}{v}$, $u = 6s + 7t$, $v = 7s − 6t$
my asnwer is
$$\frac{\partial z}{\partial s}= \frac{6\sec \left(\frac{u}{v}\right)^2}{v}+\frac{7u\sec \left(\frac{u}{v}\right)^2}{v^2}$$
$$\frac{\partial z}{\partial t}= \frac{7\sec \left(\frac{u}{v}\right)^2}{v}+\frac{6u\sec \left(\frac{u}{v}\right)^2}{v^2}$$
But the system said I am wrong. Anyone can help me check? thanks!