D is the midpoint of line BC which means BD and CD are equal.
Set the two equations equal and solve for x:
3x-3 = x+5
Subtract 1x from each side:
2x-3 = 5
Add 3 to each side:
2x = 8
Divide both sides by 2:
x = 8/2
x = 4
Now we have x, replace x with 4 and solve for BD:
BD = 3(4)-3
BD = 12-3
BD = 9
