The sum will have an x-term sumtimes ... whenever
the x-terms alone don't add up to zero.
Examples:
-- Two linear expressions:
y + x
and
y + 2x
The sum of the x-terms alone is (x + 2x) = 3x .
So the sum of the expressions has an x-term.
(y + x) + (y + 2x) = 2y + 3x .
_______________________________
Two different linear expressions:
y + 3x
and
y - 3x
The sum of the x-terms alone is (3x - 3x) = zero.
So the sum of the expressions doesn't have an x-term.
(y + 3x) + (y - 3x) = 2y .
