Secant Method (Numerical Analysis)

Hello,

I'm having trouble with the following past exam question.

A fisherman wants to set the net at a water depth where the temperature is 40 degrees F. By dropping a line with a thermometer attached, he finds that the temperature is 38 degrees at a depth of 12 meters, and 46 at a depth of 5 meters. Use the **Secant Method** to determine a best estimate for the depth at which the temperature is 40.

Now from my calculations the equation that fits these points is

$\displaystyle f(x) = \frac{7}{8} x - 45.25 $

so I know that the depth at which the temp. is 40 is 10.25m

The thing that Im having a problem with is how to solve it using the Secant method.

The secant method is easy enough to apply if I am asked to find the root of an equation with initial guesses $\displaystyle x_0 \ and \ x_1$.

But Im not trying to find the root here so I am lost (Doh)

Any ideas?