Find the following derivatives. \( z_{s} \) and \( z_{t} \), where \( z=7 x y-2 x^{2} y, x=2 s+5 t \), and \( y=2 s-5 t \) \( \frac{\partial z}{\partial x}=7 y-4 x y \) (Type an expression using \( x \) and \( y \) as the variables.) \( \frac{\partial x}{\partial s}=2 \) (Type an expression using \( s \) and \( t \) as the variables.) \( \frac{\partial z}{\partial y}=7 x-2 x^{2} \) (Type an expression using \( x \) and \( y \) as the variables.) \( \frac{\partial y}{\partial s}=2 \) (Type an expression using s and \( t \) as the variables.) \( \frac{\partial x}{\partial t}=\square \) (Type an expression using \( s \) and \( t \) as the variables.)