H- + C6H5CH2COOH = H2O + C6H5CH2COO-

What is the K for the reaction above in terms of Ka's, Kb's, Kw etc.?

Did you make a typo? Did you intend this to be

OH^- + C6H5CH2COOH ==>H2O + C6H5CH2COO^-
This looks like Ka/Kw to me.