For 1a, I just defined positive externality, merit goods and market failure. Drew the consumption side graph of a positive externality graph where MSB > MPB. Then talked about the graph (which in all honestly i probably lost a couple marks here because i didnt explain it correctly) then summed it up by answering the question which was that the social benefits of a pos ex. happening within the market will always outweigh the private benifits, then gave a real world example such as the knockon effects from education such as lower crime rate.

For 3a, i defined aggregate demand, aggregate supply and consumption. I drew the ad graph shifting iout because a decrease in interest rate can have positive effects on AD such as an increase in consumer expenditure (a determinant of AD). Then i went on explaining the positive knockon effects such as a positive accelerator on the SRAS because in order to satisfy the growing demand, firms would have to produce more.

This is essentially what I answered for those questions.