In the first one, what's the opportunity cost of the units not produced?

Trade off : Trade off simply means that you are compromising between the two things. No better can be done without worse. If we want to make one side of a particular thing good, it must come at cost of other. e.g. Think of you only 1 hour to study two subjects and you have enough syllabus to do. You do nothing of syllabus of both, you dont have any prior knowledge of the subject also. So, if you read more about one subject in one hour, it must come at the cost that you read lesser of the other subject during the same hour.

Opportunity Cost: The cost of next best alternative of a situation is known as the opportunity cost. Thus, you had next best option available and you do not chose it, the cost of not choosing this option is called the opportunity cost. Suppose, i have one hour to do extra things other than study. Suppose i have two options To play cricket and other is to bet some money in that one hour and earn say $300. Suppose as per my utility i prefer to choose playing cricket, then opportunity cost for playing cricket shall be the cost of next best alternative not chosen up i.e. the money you choose not to earn i.e. $300.

Equity: Equity means that every economic agent in the economy gets the same economic benefits. This is something more related to the real world. What institutes like World bank, IMF are doing is to bring equality so that over the globe people are treated equally. In order to achieve that, they are running a lot of development programs.