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Notice that the given triangle is a scalene triangle: it has no congruent sides.
Since 3 times 7 is 21, the side of the length 7 can not be longest.
If the side of the length 7 is the middle length side, then there is the shorter side;
but then the third side has the length greater than 30-2*7 = 16.
It contradicts to the triangle inequality.
Thus the side of the length 7 is with necessity the shortest side of the triangle.
In any triangle, the shortest side is opposite to the smallest angle.
Hence, the side of the length 7 in the given triangle is opposite to the angle C.
Solved.
