First, please write the problems unambiguously. If the "2" in "7x2" is supposed to be an exponent, write it as "7x^2". And if the "x" in "dz= zxdx+..." is a subscript, write it as "dz= z_x dx+...".

Now, what have you done yourself? If z= 7x^2y^e (and is that really an exponent of "e", not, perhaps, 3?), then what are z_x and z_y?