nprofile1qy2hwumn8ghj7un9d3shjtnddaehgu3wwp6kyqpq0m39fudmqx3tj2l9vsstk8tpvxehzh79wyvawem4ltewlc332c5q00rfqu (nprofile…rfqu) Thanks for your message.
There are about 40 questions in this example, and my son says that they are all very basic, about simplifying expressions. It is just this question that wasn't clear. He usually gets all questions right, and gets very bored. So this one helped him to be less bored, but the way of getting kinds less bored should not be by asking an unclear question. I don't "the context" was explained in the lessons for this particular question.
In any case, usually unknowns are x,y and a,b,c are constants, when the quantifiers are omitted in informal mathematical language (which they were here), and in this case it is the other way round, with the additional fact that the unknowns a,b,c, don't depend on the data x,y.