let the other 2 unknown sides be y n z
look at the two smaller triangles inside
y^2 = 12^2 + x^2 and
z^2 = 21^2 + x^2
from the biggest triangle
(12+21)^2 = y^2 + z^2
substituting
(12+21)^2 = (12^2 + x^2) + (21^2 + x^2)
33^2 = 12^2 + 21^2 + 2x^2
2x^2 = (33^2 - 12^2 - 21^2)/2 = 504
x^2 = 252
x = 15.9
ans is B
