6. Note that $z=f(x,y)$ where $f$ is differentiable, $x=g(t)$, and $y=h(t)$. It is known that $g(3)=2$, $g'(3)=5$, $h(3)=7$, $h'(3)=-4$, $f_x(2,7)=6$, and $f_y(2,7)=-8$. Determine the value of $dz/dt$ at $t=3$.

(A) 62

(B) 30

(C) 32

(D) -2

(E) 1。