Use polyval to evaluate the derivative at xder. This will be the
% slope of the tangent line, "a" (general form of a line: y = ax + b).
% 4. Calculate the intercept, b, of the tangent line. Note that given x =
% xder, you can calculate y at xder. Then, since "a" is already known, you are left
% with one equation and one unknown, b.