Piecewise Function Models, and Curve Approximations

Say we approximate the function $f(x) = x^2$ 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 equation for any one of these lines, in terms of $k$?