Um okay, from memory the question was asking how much the total subsidy would be costing the government overall. For example if quanity was 400 and subsidy was 2$ then it would cost the government 800$ however for the question i had to work out the quantity except im not how to work it out looking at a grap
Ahh ok, In that case, I'm pretty sure the gap you're talking about would just be Q1-Q from the graph that I previously included. If suppose they said that the subsidy costed $1000 in total and that before the subsidy they were supplying 50 units at $10. The subsidy will shift supply to right as the price falls to $5, find the quantity that the economy can now supply (I'm just making this question up I have never actually seen this in a practice paper)
Subsidy is the area of the rectangle caused by the shift, so the width would be $10-$5 =$5;
Then dividing by $1000 gives 200 (the length) = the new quantity that the economy can supply
i.e economy can now supply 200 units of the good -> Increasing by 150 units from the 50 units of the good pre-subsidy
I hope this is what you're asking- sorry about the confusion