To start, use the fact that the largest angle is the opposite of the longest side. Now, use the Law of Cosines (a^2 = b^2 + c^2 - 2bc cos A), where a is the longest side and A is its opposite, and b and c are the other sides. Plug the values in to get 1681 = 1962 - 1638 cos A, or cos A = 281 / 1638. Now, use a calculator to get that A is approximately 80.12 degrees, or 80 degrees.
