How to view this algorithm as a tridiagonal system? (Curve shortening flow)

Question

I'm trying to create an implementation of a numerical method to approximate curve shortening flow, following "Computation of geometric partial differential equations and mean curvature flow" by Deckelnick, Dziuk and Elliott. In one step of their algorithm, with $X_0^j$ defined (for $j=0,\cdots,N$) they say to compute $X_j^{m+1}$ (again for $j=0,\cdots,N$) from the tridiagonal systems $$(q_j^m+q_{j+1}^m)(X_j^{m+1}-X_j^m)-\left(\frac{X_{j+1}^{m+1}-X_j^{m+1}}{q_{j+1}^m}-\frac{X_j^{m+1}-X_{j-1}^{m+1}}{q_j^m}\right)=0,$$ where $$q_j^m:=|X_j^m-X_{j-1}^m|.$$ I am having trouble understanding how this relates to the typically notated tridiagonal system so that I may solve it by the standard algorithms. Any tips on how to view/write it as such? (Sorry if this is a bit heavy or vague).