Well, in general the formula for elasticity will be 1/slope * P/Q for the point slope formula, or for the arc elasticity it is [(change in Q)/(average Q)] / [(change in P)/(Avearge P)]. I don't think either of them can be used for your problem. This picture describes this problem, hopefully it helps. Basically it shows good x on the x axis, good y on the y axis, and showing the budget constraint shifting from the outer curve to the inner curve after the price change. It also shows that the amount of good y consumed does not change.