Use the Law of Cosines.

C² = A² + B² − 2ABcos(c)
(x)² = (15)² + (21)² − 2(15)(21)cos(40°)

$x=\sqrt{666-630\text{cos}(40\degree)}$

x ≈ 13.542

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

x ≈ 68.770