S function builder (Internal computed Parameters)

Hello, I am using Matlab S function builder in the Simulink. can anyone tell me how can I define internally computed parameters or states? for example: suppose a block has two parameters, the volume, and density of some object, and the output of the block is a function of the input signal and the weight of the object. In this case, the weight can be viewed as a third internal parameter computed from the two external parameters, volume, and density.

How can I define the internally computed parameter?

Regards, Ameer