The answer would be E= 27 miles
You would start out by figuring out how many hours it would take him to swim by dividing 6/2 to get 3 then you would find out how long it would take him to run by dividing 15/6 to get 2.5 and you would add that to the 3 to get 5.5 and subtract that by 7 to get 1.5 which is how long he will have to complete leg 3 so you would take 18x1.5 to get 27 mph
The "m" in y = mx + b is the <u>slope.</u>
It is the number of units a point goes up, down, left, or right each time. Making the line linear/straight.

"rise" is the the number of units you go up(+) or down(-), "run" is the number of units you go to the right
For example, if your slope is:

You are going up 1 unit, and to the right 2 units
3 or 
You are going up 3 units, and to the right 1 unit

You are going down one unit, and to the right 2 units
-3 or 
You are going down 3 units, and to the right 1 unit
The answer is E because during 5:00-9:00 the temperature didn’t change