To summarize how to write a linear equation using the slope-interception form you
Identify the slope, m. This can be done by calculating the slope between two known points of the line using the slope formula.
Find the y-intercept. This can be done by substituting the slope and the coordinates of a point (x, y) on the line in the slope-intercept formula and then solve for b.
Once you've got both m and b you can just put them in the equation at their respective position.
So to find average speed, add the distance and divide it by the total time. So the total feet is 704+328=1032 feet in 8+4=12 seconds. So to find the average speed, then the average speed is 1032/12=86 feet per second