I'm sorry if this is in the wrong place...also that I don't know how to format this so it looks pretty

I have an equation that is a footnote in my thermodynamics book:

(dx/dy)z = -(dx/dz)y * (dz/dy)x

where d is the partial derivative and the x,y,z outside of the parenthesis denote keeping constant (ie the LHS reads the partial of x wrt y with z constant)

is there an analogous formula for having 2 variables to keep constant?
i.e. (dx/dy)z,a ?


Thanks for any help!