Help Please

[BCD] = 100/3.

what do the square brackets mean?

Hey Melody! The square brackets usually represent area.

We know that the area of ABC is 10, so its height to AB  from C must be 20/6=10/3. This same height is the height to BD from C, so we can compute (10*10/3)/2=50/3. 

Guest, you were close, but I think you forgot to divide by two at the end!

Hope this helped!