Roll yield with a moving curve

(a) Derive (10.12): the one-period excess log return of a futures position with fixed time-to-maturity $\tau$ is $\Delta\ln S - c_t\Delta + (c_{t+\Delta} - c_t)(\tau - \Delta)$. (b) Evaluate it for $\tau = 2/12$, $\Delta = 1/12$, $S_t = 7000$, $c_t = 12\%$, $S_{t+\Delta} = 7100$, $c_{t+\Delta} = 20\%$. Check by computing the two futures prices directly. (c) Which term would a calendar-spread trader be trading?