Write the slope-intercept form of the line through the points (4,-5) and (2, -1).

Slope intercept form is y=mx+b
to solve for m (slope) you must first subtract y2-y1 and x2-x1 then divide your answers

-1 - -5 = 4 (y)
2 - 4 = -2 (x)
4/-2 = -2

so your slope is -2

next plug in your slope and the point you choose

-1 = -2(2) + b
or
-5 = -2(5) +b

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