That's what you just did!

That is exactly what the "secant" method is. Given an equation, f(x)= 40, and two points, x and y, such that f(x)< 40< f(y), you construct the straight line from (x, f(x)) to (y,f(y)) and solve for the point, z, on that line that gives a value of 40. If f(z) is not 40, it is either above it or below it so you repeat with this new point.

Here, you are not given "f" but you are given the first two points. The only thing you can do is use the secant method to find "z" for the first step. And that's what you have already done.