Answer:
a =

Explanation:
First, you want to add d on both sides.
ab-d = c
+d +d
ab = c+d
Now, divide b on both sides.
ab = c+d
/b /b
a =

I hope this helps; have a great day c:
(Also, I just realized that I solved only the question that you typed, not the picture...do you need the answer for the question in the picture???)