AM must be a half of AB, because M is the midpoint of AB.
AM = 1/2AB
2x + 9 = 1/2(8x - 50)
Distribute 1/2 to (8x - 50).
2x + 9 = 4x - 25
Subtract 2x from both sides.
9 = 2x - 25
Add 25 to both sides.
34 = 2x
Divide both sides by 2.
x = 17
Now plug x into AM = 2x + 9.
AM = 2(17) + 9
AM = 34 + 9
AM = 43
