The mixed number is 4 5/12.
The average speed of Joshua during that time is 2500 m/h.
Explanation:
It is given that Joshua started cycling at 5:15 pm. By 8:09 pm he has covered a distance of 7250 m.
The total time taken by Joshua from 5:15 pm to 8:09 pm is

Dividing we get,

Adding, we have,

Thus, the total time taken by Joshua is 
To determine the average speed we use the formula,

where
and 
Hence, substituting the values we have,

Dividing, we get,

Thus, the average speed of Joshua during that time is 2500 m/h.
You just need to try with all of the pair f.ex
0,-2 -> put 0 in the x-> y=8*0-2=-2 so ok it is q possible pair of the function..
You try every pair till you find set where all the pairs fit
So set 1
1 and 6: 8*1-2=6 ok
2 and 14 : 8*2-2=14 ok
Hope I could help