What is the length of midsegment XY

(note i put the graph in so plz )the graph shows the amount of water that remains in a barrel after it begins to leak. the variable x represents the number of days that have passed since the barrel was filled, and y represents the number of gallons of water that remain in the barrel. what is the slope of the line

What would the rule be for the reflection of y=-3

Determine which postulate or theorem can be used to prove that lmn =nmo