I am choosing to approach this question a little differently. Building another city does not make a ton of sense. It is just going to be really expensive and make more carbon emissions. On the other hand, improving a city would be less expensive and cause fewer carbon emissions. Some things you could do is give people tax cuts for having solar panels and electric cars. Make it cheaper for people to go eco-friendly. For example, do not charge for reusable bags and straws. Do not give people a straw at a restaurant unless they ask for one and add it to their bill. Make it harder for people to be mean to the environment and make it easier for people to be nicer. There may be some outrage at first but people will adjust pretty quickly if there are benefits to going green. Setting a good example could inspire other citys and that could turn into counties and cause a domino effect of a healthy environment.

I think that the exercise here is supposed to just forget the costs of building an actual city but focus on the smaller programs that could help the city reduce carbon emissions. By in-large though you seem to answer the questions, and well at that. Making it cheaper for people to have eco-friendly items and setting examples for other cities are undoubtably important programs to run. I think that making solar panels cheaper to access in particular as well as increasing the use of public transportation in the hypothetical city. These could help reduce carbon emissions if many cities implement these types of programs.