solve block problem

You may use the normal assignment operator to assign a variale the result of a symbolic evaluation. Or is the question meant how to get the positive result only?

You have not used a solve block, as your subject pretends. You wouldn't experience the problem if you had used a solve block as you could use constraints in a solve block.

With the symbolics you may either use "assume" to tell muPad that all variables (units are seen by the symbolics as unknown varaibles, too) are positive

or you may assign the symbolic result to a variable and then pick the correct solution yourself using the vector index