4a - 5b + 6ab - 8b - 3a =

Write the thing spaced out:

4a - 5b + 6ab - 8b - 3a

The signs (+,-) are assigned to the numbers it is in front of.

We minus the "like" terms:

4a - 3a = 1a

-5b - 8b = - 13b

+6ab = + 6ab

Put them all together:

1a - 13b + 6ab

The answer for, "4a - 5b + 6ab - 8b - 3a" = "1a - 13b + 6ab"