The answer is C 7
Hope this helps
As given m is a midpoint of ab. So am is half of ab.
So we can write

As given am = 3x+3 , ab = 8x - 6.
So





So x = 6.
So length of am = 3x+3 = 3*6 + 3 = 21
I would rather use a ruler but it depends on what you are doing.
Answer:
If your answer is multiple choice, try to look for a graph that matches these intercepts. I hope this helped you. Have a great rest of your day!