Answer: y(k) = 75.32*k + 1986
Steps:
Orbital period of the Halley's comet is 75.32 years. Set up a function with variable k denoting the "k-th" time the comet is seen again since last sighting in 1986:
y(k) = 75.32*k + 1986
The function value is the year (including decimal fraction) when the comet will be seen for the k-the time since 1986.