Let the equal side lengths be a.
14² = a²+16²-2*16acosb
196 = a²+256-32acosb --- (1).
21² = 2a²+2a²cosb
cosb = (441-2a²)/2a² --- (2).
Substituting (2) in (1).
196 = a²+256-32a(441-2a²)/2a²
32(441-2a²)/2a = a²+256-196
32(441-2a²)/2a = a²+60
14112-64a² = 2a³+120a
2a³+64a²+120a-14112 = 0
It implies;
a = 12 units.
a is the required length.
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