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Economics English Problem with math

Hi, I am not sure if this is in the right area, however, I have an economics problem that involves mathematics. For the most part, I do believe this is simple math by just taking the secant slope of the two points, however, they thought us how to do partial derivatives and I am not sure if they claim that it always simplifies to -x2/x1 because they were not clear on the issue and we were not taught partial derivatives in multi-variable calculus yet. The question with the answer I think it is listed below. However, since they taught us the partial derivatives way which is to give you the exact Marginal Rate of Substitution (MRS) I think it may be |-x2/x1| however, I do not believe English is clear enough to know which method they prefer.

Thanks.