Before you read this!! To me, it looks like 8 is the entire length of lineJL, but it may only be the length of lineJM. I've written the steps for lineJL=8 NOT lineJM=8. If this is not the case and 8 is only the length of lineJM let me know so I can fix my answer!
Explanation:
The area of a triangle is A=1/2(b)(h)
B=base
H=height
Your height is found in the line in the middle of the triangle, for you it is 6.
The base is the long bottom line on the triangle (lineJL), for you it is 8.
So let's put it in.
A=1/2(8)(6)
A=24sq units (C)