The equation of the line passing through the ordered pair (a,0) and parallel to the line x+2y=7 is :

With a given parallel line and a given point on the line
we can use the point-line method:  A(x-x0)+B(y-y0)=0
where
Ax+By=k is the given line, and
(x0,y0) is the given point.

Here
A=1,B=2,k=7.
(x0,y0)=(a,0)
=> the required line L is given by:
L:  (x-a)+2(y-0)=0
on simplification
L: x+2y=a