Anything you can do is easy. Anything you cannot do is difficult -- until you learn how to do it.
This defensive problem would defeat many players because they play by using rules, rather than by thinking. You are East. South is in three no-trump. Your partner leads the diamond two. How would you plan the defense?
After you have decided, look at the full deal and critique the bidding.
I like the auction. However, it would be reasonable for North to rebid two spades, showing his six-card suit. But his doubleton Q-J of hearts is as good as three low cards, and partner will not be expecting more, because with three hearts, North would have raised immediately to two hearts, not responded one spade. South, with 17 points, makes a try for game with two no-trump. North should accept with eight points and can take the opportunity to show his extra spade length in case South has 2-5-2-4 distribution.
Many defenders would win with the diamond ace and thoughtlessly return the diamond seven. South would then take 10 tricks.
West's fourth-highest lead tells you that South has three diamonds and, therefore, 1-5-3-4 shape. South is marked with some 17 high-card points. If West has the K-J of diamonds, declarer has the blank spade ace. After losing four diamond tricks, South will surely take the rest.
You should win trick one and shift to the spade three, playing partner for ace-doubleton. Do not start spades with your king, which would block the suit.