The total expenditure of a household is partly constant and partly varies directly as a size of household find the equation connecting d and s if when Q=300 and when d=400 and s=6

Answer 1

what are d and s, and where does Q come in?
I guess s is the size, but what is the expenditure?

It seems you want an equation like
expenditure = const + variable * size
That is, something like y = b+mx, but with the names changed as needed.
You will need two data points to determine the constant and the variable