Iwill give brainliest to whoever is correct

Because M is the midpoint of AB, then AM and MB are equal distances. And because a segment can be written as the sum of its pieces, AM + MB = AB.

So,

AM + MB = AB                  <--- distance of a segment is the sum of its pieces

AM + AM = AB                  <--- M is the midpoint, so AM = MB

3x + 3 + 3x + 3 =  8x - 6   <--- substituting known amounts that were given

6x + 6 = 8x - 6                  <--- collect like terms on the left side

6x = 8x - 12                       <--- subtract 6 on both sides

-2x = -12                            <--- subtract 8x on both sides

x = 6                                  <--- divide both sides by -2

Because x = 6, we put it back into AM. 3(6) + 3 = 18 + 3 = 21

Thus, AM is 21 units.