We know that the initial temperature is -3 then it rises to 8 and drops by 6 degrees then you just need to subtract six from 8 
Aren’t you supposed to multiply the exponents to where it’s 5^24 ?
Equation of the line: y=4/5x-18/5
To find slope subtract the two points in the coordinate (y). We have 8 and 4 so subtract those two. You get 8-4=4. Now start from the same coordinate expect this time subtract (x). Which is 9-3=6. You have 4/6 simplify and you get 2/3. Your slope is 2/3. Hope this helps