Let L: (1+20/3a, 2-4/3a, -5-28/3a) + t(-1,2,1) and M: (-4,3,2) + u(-4,-3,-4) be 2 lines in R^3. Find the value of a that ensures M and L intersect.

I'm stuck on this. So far I have:

1 +20/3a - t = -4-4u

2-4/3a -t = 3-3u

-5-28/3a + t = 2 - 4u

but I dont know how to take it forward. Anyone able to help?