Its important to learn the difference between ranges and vectors!
a is a range variable, not a vector even if the evaluation of a or a+3 might look like a vector (which was not a good choice by Mathcad anyway).
A range variable may be seen as kind of an implicit for-loop.
Range variables should be used in a limited way only
- indexing and accessing vectors and matrices
- as the independent variable in plotting a function
- in programs at for-loops
Create a as a vector and your assignment will work.
The problem exist since the beginnings of Mathcad and always was a pitfall for new users. I attach a pdf of two files by Stuart Bruff about Vectors vs. Ranges.