Use the Law of Cosines.

$\left(x\right)^{2} = \left(15\right)^{2} + \left(21\right)^{2} − 2\left(15\right)\left(21\right)\operatorname{cos}\left(40°\right)$

$x=\sqrt{666-630\text{ cos}(40º)}$

$x ≈ 13.542$

Find the last angle, then use the Law of Sines.

$x ≈ 68.770$