Try to visualize me.

1)Draw parallelogram ABCD.

2)Draw bisector of A and let it intersect at E.

3)Now <BAE = <EAD (because it is a bisector).

4)But <EAD = <BEA (parallel postulate, in simpler terms, alternate interiror angles).

5)Thus, <BAE = < BEA

6)Thus, triangle ABE is isoseles with AB = BE.

7)But AB=6

8)Thus BE=6

9)Thus, BC=2(BE)=2(6)=12

Half of that.2. Given triangle ABC with a perimeter of 25, what is the perimeter of the smaller triangle made by the lines of the midpoints

Because each length of side is half the corresponding one to the larger triangle.