Thera appears to be some incoherence in the instructions format written in the book/lectures and in the instruction format in the summery provided in the test itself.
On the first one the argument registers come before the destination register and on the latter it is the opposite. I hope that until the test we could get an answer about the matter.
The definition given in the book is the standard you should use.
That being said, in this question you need to implement a new instruction which is not part of the existing ISA. For such new instructions, the order of the inputs/outputs doesn’t need to be the same as for existing instructions.