Hydrostatic Relation Empirical Formulas

An empirical formula relating pressure and density for seawater w/ temperature held constant is:
p/pa = (k+1)(q/qa)^7 - k
where p= pressure
pa = pressure at the surface
k = dimensionless constant
q = density
qa = density at the surface
Using the formula in the hydrostatic relation, determine the pressure as a function of depth.
The hydrostatic relation is: p+dgz=constant
I am uncertain as to the proper approach for beginning this problem. I know that if I wanted to take the derivative of the empirical formula there must be a z term. I tried to insert P/RT in for q (q=P/RT) and then substitute for T with the relation T=T0-kz. But I wasn't for sure where to go from there.

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