this is really confusing to me

Since it is given the point E is the midpoint of segment BD then we can also say that it is the midpoint of segment AC.

So since E is the midpoint we can set AE and EC equal to each other and solve for x.

AE = EC

6x - 15 = x + 30
6x = x + 45
5x = 45
x = 9

So the value of x is 9.

Hope this helps :)