If the line passing through the points (a, 1) and (6, 5) is parallel to the line passing through the points (9, 6) and (a + 7, 1), what is the value of a?

I know that they need to have either the same slope, but I have no idea how to do this problem

My work

5-1/6-a = 4/6-a

1-6/a+7-9 = -5/a-2

After that...I'm lost