The longest possible value of x is the one that make the "obtuse" angle equal to 90°. By the Pythagorean theorem, ...
20² = x² + (3x)²
400 = 10x²
40 = x²
2√10 = x
The greatest possible value for x is 2√10.
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x cannot actually equal this value, but may be a value as near to this as you like.
