|AM| = |MB|
|AB| = |AM| + |MB| ⇒ |AB| = 2|AM|
|AB| = 8x - 6; |AM| = 3x + 3
subtitute:
8x - 6 = 2(3x + 3) |use distributive property: a(b + c) = ab + ac
8x - 6 = 6x + 6 |add 6 to both sides
8x = 6x + 12 |subtract 6x from both sides
2x = 12 |divide both sides by 2
x = 6
|MB| = 3x + 3, subtitute
|MB| = 3 · 6 + 3 = 18 + 3 = 21