@tomleslie 

I also checked with the 2017 version, everything is fine, unlike the 2021.2 version.

@tomleslie 

Thank you so much for this great sharing. Personally, I'm tired and frustrated with version 2021.2 (the latest version of Maples)

@Kitonum 

I am using version 2021.2. If it is an error from version of software to be it's true that this version has quite many negative issues (I have also found another issue in using of "back solving assistant", recently.) Thank you so much again.

@Kitonum 

When I solved the problem with your commands, I got a single solution instead of 2, like yours, and hence an error message. Can you point out why?

@Kitonum

Thank you a lot for your help! 

@Kitonum 

I found the reason of problem. The cause of the problem is that in version 2021.2 that I am using, "vector" command  has been superseded by "Vector" command. Thank you a lot again!

@tomleslie

 I can do that with your way of convert. Thank you very much.

@Kitonum 

Thank you a lot. I already use with(LinearAlgebra), however I still can't

@Kitonum 

Please help me! After getting the vectors by converting as above, I can't use them in calculations like normal vectors, for example can't calculate the directed product between 2 vectors with CrossProduct command. Why?

@Kitonum

Thanks to your guide, I did it already. Thank you very much! 

@tomleslie 

Thank you a lot.

@tomleslie 

Thank you very much! I am a newbie to MAPLE software. I am learning to use it.

@mmcdara 

The problem is that it is not possible to enter (or paste) a formula into it. A message appears stating that the program needs the entered formula to be an XML-Document or XML-Element (?!). I don't know how to get them.

@mmcdara 

Thank you for your enthusiastic guide. Unfortunately, however, the problem is still unresolved. The version of Maple that I am using is Maple 2021.2. In this version, "Back-Solver" is not listed under "Assistant". Instead it has the path as: Tools - Tutors - Bascis - Back-Solver. I have tried many times trying to "copy-paste" one of the 3 suggested formulas. But all receive the error message "...invalid input...."