please help if you know trigonometry ;)

Answer:
12.73m
Step-by-step explanation:
using Pythagoras theorem
a = x (i change x to a for easy reference due to multiplication)
18 x 18 = a x a + a x a
324 = 2a^2
a^2 = 324 / 2= 162
a = [tex]\sqrt{162}[/tex] = 12.7279 = 12.73 (2 decimal place)