Help with finding constant values

I am supposed to "Prepare Simulink models that will take the value of b from a “Constant” block". The equation is θ'' + bθ' + sinθ = 0. So far, I have this set up

but I obviously cannot run this simulation with a variable in the constant block. How am I supposed to find the value of b?

[Arvind Narayanan]

The value of b needs to be input by the user. The equation remains valid for different values of b. Hence, there is no fixed answer.