The middle one i tried it but idek can't really explain it without the picture

(6,1) (-3,-5)

Using the 2 co-ordinates we can find the value of the slope using the formula of

slope=
m= 
   = 
   = 
   = 

Using the slope and one co-ordinate the y-intercpt can be be found

Using the slope intercept form y= mx+b

y= 2/3x +b 

Then plug in a co-ordinate and solve for b (in my case (6,1)) [x=6,y=1]
y= 2/3x +b 
1=2/3(6)+b 
1=4+b
b= 1-4
 = -3

There fore the y-int. is -3

If the diameter is 8 than the radius is 4 because 2r or 2*radius equals the diameter. 4*2=8. 8/2=4.