First, I would switch the sides of the equation around.
15c + 3/6 = -12
Then, simplify the equation.
15c + 1/2 = -12
Now, we need to get 15c alone since it carries the variable. We can do this by canceling 1/2 out by subtracting 1/2. Remember what you do to one side you must do to the other.
15c +1/2 = -12
-1/2 -1/2
↓
15c = -12.5
Next, get c by itself by dividing both sides by 15, because what you do to one side you must do to the other .
15c ÷ 15 = -12.5 ÷ 15
↓
c = -5/6