Since the equation is that of a line, its least and highest possible 
y-values are at the endpoints of the interval.

We substitute the left endpoint of the interval,
 for  in






Then we substitute the right endpoint of the interval,
 for  in






And the largest value of y is 60 and the least value of y is 15.

-------------------------------------

And also solve for d



Use the distributive principle:







Then cancel:



and all that's left is:



Edwin