$$V_{OC}=V_S\frac{R_V}{R_V+R_S},\;\;\;\;\;I_{SC}=\frac{V_S}{R_S+R_A}$$

Solving these for $V_S$ and $R_S$ we get:

$$R_S=R_V\;\frac{I_{SC}R_A-V_{OC}}{V_{OC}-I_{SC}R_V},\;\;\;\; V_S=V_{OC}\;\frac{I_{SC}(R_A-R_V)}{V_{OC}-I_{SC}R_V}$$