The slope of this line is
Now, differentiate the function to get
At x=-4, the value of the derivative is .
Now when is ?
Once you have a, then plug the point into the function to find the value of the function at this point [which I will call ].
Then, take this point and plug it into the equation for the line:
, where is the point on the function (and on the line)
Then solve for b.
Can you take it from here?