Secant method constant of convergence

Hi, I'm trying to find the constant of convergence for an assignment in school.

I've made a function in matlab which finds the roots with the secant method:

function [x,y] = sekant(f,x0,x1,tol,n)
x(1) = x0;
x(2) = x1;
y(1) = feval(f,x(1));
y(2) = feval(f,x(2));
      for i = 3 : n
        x(i) = x(i-1) - y(i-1)/((y(i-1)-y(i-2))/(x(i-1)-x(i-2)));
        y(i) = feval(f,x(i));
        if abs(x(i) - x(i-1)) < tol
            disp('The Secant method has converged!');
            break;
        end
      if i == n
            disp('Zero not found!');
        end
    end
data = [x' y'];
disp(data);

f = @(x) x.^2 - 0.7*x - 0.3;
tol = 0.0001;
x0 = .3; x1 = 1.5;
n = 10;

Sekant(f,x0,x1,tol,n)

Now I'm asked to find the constant in the following equation

where k > 0 & 1 < a < 2

In the assignment it says that this can be done graphically but I don't know how to do this.

Can someone give a hint how to do this in matlab?

Thank you in advance