I honestly have no idea how to solve this question

8c=5c+21

Well, subtract 5c from both sides, so cross out 5c and do 8c-5c which is 3c. S 3c=21. 21 divided by 3 is 7. So c is. 7.

Remember this rule: Whenever you try to isolate the variable do the same operation to both sides

Using the rule above subtract 5c from both sides getting 3c = 21

Divide by 3 to both sides

Now you have c = 7

Thats you answer, if you have anymore questions just PM me