Secant method to find root

I need to find how long it will take to find an object to hit the ground using secant method.

The equation is

   y=(1/k)ln[cosh(t*sqrt(gk))]

while g = 9.81 and k = .00341 t = time in seconds and y is in meters

I need to determine how much time (t) it will take to fall 1000 meters to within 1cm.

Here is what I have which is way off I'm guessing:

clc;

deff('y = f(x)','1000-(1/k)*log(cosh(t*sqrt(gk)))');

t = linspace(0,10,1000)

g = 9.81;

k = .00341;

plot(t,f(x));

r = fsolve(0,f);

disp(r);

[Walter Roberson]

What is deff() ?