What is the point slope form of the line with slope −3/7 that passes through the point (5, 8)?

They are asking for point slope form, so we need to grab the equation:
y - y1 = m(x - x1)

Then just plug in what we already know:
m = -3/7 , y1 = 5 , x1 = 8

y - 5= -3/7(x - 8)

Then solve for Y

Final y = -3/7x + 71/7

Assuming he's going 12 miles per hour, and 10 miles per hour.

Firstly he rode his bike for 2 1/3 hours at 12 miles per hour, 
12 miles = 1 hour
24 miles = 2 hours
4 miles = 1/3 hour

He rode 28 miles in the first 2 1/3 hours.

Secondly, he rode his bike at the speed of 10 miles per hour for 1 1/2 hours
10 miles = 1 hour
5 miles = 1/2 hour

He rode his bike for 15 miles for the next 1 1/2 hours, together he rode his bike for 43 miles (28 + 15).