Isolate the c. Note the equal sign. What you do to one side, you do to the other.
First, add 5c to both sides
-5c (+5c) + d = 2c (+5c)
d = 2c + 5c
d = 7c
Next, isolate the c. Divide 7 from both sides
(d)/7 = (7c)/7
d/7 = c
c = d/7 is your answer
hope this helps