Will the sum of two linear expressions each with an x-term , always, sometimes, or never have an x-term ? explain your reasoning

Sometimes it will, and sometimes it won't.

I reason as follows:

(21 + x)  +  (30 + 2x)  =  51 + 3x      has an 'x' term

(42 + x)  +  (30 - x)  =  72                  has no 'x' term.

3x+9+4x+x=8x+9that's the answer for your question.