Answer:
- not possible: 1, 2, 4, 5, 8, 11
- possible: all others
Step-by-step explanation:
Obviously, all multiples of 3 and 7 can be paid.
1 cent can be added by adding a 7-cent stamp and taking away two 3-cent stamps. 2 cents can be added by adding three 3-cent stamps and taking away one 7-cent stamp. Hence any value more than 7 +2(3) = 13 cents can always be accommodated.
Amounts that are impossible:
1, 2, 4, 5, 8, 11 cents
All other positive integer amounts are possible.
For the limit approaching 3 from the right, you want to follow the line to the right of x = 3. From the graph you're describing it sounds like that's y = -3.

The RHS limit is -3 even though f(3) = 7
Answer:
6x-5 if x<5
Step-by-step explanation:
If x<5, then x-5<0.
If x-5 is negative, then |x-5)=-(x-5) or -x+5.
5x - |x - 5|
5x-(-x+5) provided x<5
5x+x-5 by distributive property
6x-5 combined liked terms