What does it mean when a graph is positive or negative?

Which expression can be used to determine the slope of the line that passes through the points -6, three and one, -9

Saline iv fluid bags cost $64.20 for 24, 1000 ml bags from one supplier. a new supplier sells 500 ml bags for $2 each. the hospital uses about 10,000, 1000 ml bags each month. nurses report that about half of the bags are more than half full when they are discarded. if you are going to use only one supplier, which size bag provides the best deal, and how much will you save each month? compare the cost of the new supplier to the current one.

What is the measure of ac? 5 units 13 units 26 units 39 units 3x- 2b 6x + 9