In a problem such as this we want to isolate our variable 'C' on one side of the equation by either adding/subtracting a constant to both sides or dividing by a coefficient. In this case, since we have C - 3.5 on one side, we can add 3.5 to both sides to leave 'C' by itself since C + 0 = C.
After doing so, we have C ≤ 3.5 + 3.5 = 7, so C ≤ 7