Piecewise Function Models, and Curve Approximations

Let $b > 1$. Say we approximate the function $f(x) = b^x$ using a piecewise linear function $g$. For all integers $k$, the approximation $g$ connects the points $(k, f(k))$ and $(k+1, f(k+1))$ with a line. What is the slope of any one of these lines, in terms of $k$?