This right triangle with 30°, 60° and 90° is called semi equilateral. Its particularity is that the side opposite to 30° is equal half of the hypotenuse.
Here the longest leg is nothing but the hypotenuse =18.(given):
Let x be the side opposite to 60° and let's calculate it by Pytaghoras:
18² =(18/2)² + x²,
324 = 81 + x² . Subtract 81 from both sides
324 - 81 = 81 + x² - 81
243 = x² and x = √243 =9√3