For this problem, you can use the right angle shortcuts.
1) solve for the triangle with sides 15 and 9 (or the height of the whole triangle)
use Pythagorean theorem: 9²+b²=15²→81+b²=225→subtract 81 from the left and right sides→b²=144→find the square root→b=12 (dotted red line).
2) now that you know the height of the triangle or the missing side of the triangles with the side lengths 9 and 15, you can solve for x using the right triangle shortcuts. This shortcut is side a (shortest side) = y, side b (second shortest side) = y√3 and the hypotenuse = 2y.
3) Since we just solved for side b (dotted line), and because this is the shortest side, we know this length is 12. The second shortest side will be 12√3 and side x the side you're solving for is 24.
-E :)