Figuring out X on a parallel line without graphing

Hello, i'm stuck on this question:

Determine the value of x so that the line segment with endpoints A(x,3) and B(1,7) is parallel to the line segment with endpoints C(2,-4) and D(5,-2)

Ok, i got the slope by using the forumla Y2 -Y1 over X2 - X1

-4 -(-2)

---------

2 -5

=

-2

-----

-3

That is the slope, now everytime i get to this point, i have no choice but to multiply, so in this case X = -5, so coordinates would be (-5, 3). But this answer was found through graphing. On a test, i won't have the time to graph, so how can i find x without having to graph it (when i have the slope already).