In part (a), you should come up with an equation of the form:
y(t) = f + gy(t-1) + hy(t-2) + G
where f, g, and h are specific numbers. The idea is to compare how the path of y will differ through time if G is changed to G+1 at time t. In the first period, period t, the new value is y(t) = f + gy(t-1) + hy(t-2) + G+1, so that ∆y(t) = +1. Continue to find ∆y(t), ∆y(t+1), ∆y(t+2), etc. and you can determine the shape of the response.
