Minimize with the constraints.
So, the minimum is 10, now what ! lol
2. Draw the corresponding lines which are the borders of the feasible region.
3. The line must contain at least one point of the feasible region and the value of C must be a minimum.
4. According to the sketch you see that
5. To get an exact result calculate the coordinates of the vertices of the feasible region and check which vertex produces the minimum value of C.