A good task to start learning lisp, look at code examples,
entsel line 1,
get midpoint line1
entsel line2
get midpoint line2
get length of line
get midpoint again of the 2 lines
angle mid1 mid2
use polar to work out the end points of the centre line
get start and end points of the lines
draw line to the ends of the lines
Have a go much better than just waiting for someone to provide an answer. Plenty here to help.