If a=4b, c= 4b/3 and d=5c/4, what is a in terms of b, c, and d???

Based on the information you have given, a = 4b. That is stating a in terms of b.

If you solve c = 4b/3 for b, you get b = 3c/4. So, you could substitute that in the original equation to get a = 3c.

If you solve d = 5c/4 for c, you get c = 4d/5. You can substitute that into our equation from above to get (after simplifying) a = 12d/5.

However, having answered this question for you in the forum I would suggest that it would be better for you to post the question in the Questions section for one of the videos in https://www.khanacademy.org/math/algebra/solving-linear-equations-and-inequalities/solving_for_variable/v/rearrange-formulas-to-isolate-specific-variables