#### Answer

a. They are parallel lines.
b. The system does not have a solution.

#### Work Step by Step

a. $x + y = 4$ and $x + y = 3$ are parallel lines.
Both linear equations are in the standard form ($Ax+By=C$).
Their slopes are calculated as $m=-\frac{A}{B}=-\frac{1}{1}=-1$.
Since they have the same slope, they are parallel lines.
b. Since, they do not intersect, the system does not have a solution.