Since the line is written in slop intercept form
we know that the slope of line is
Since we need the slope to equal the derivative we get...
solving for x we get
This gives us half of the ordered pair...
plugging this into the equation of the parabola we get
This completes our ordered pair giving us (1,-12)
Now finally we use this ordered pair to solve for c plugging the ordered pair into the equation of the line we get....
solving for C we finally end up with...
I hope this helps. Remember that the derivative represents the slope of the graph at any point.