Hi there!
[tex]\large\boxed{x = 9}[/tex]
Both bases have an angle measure of 55°, so the triangle is isosceles.
An isosceles triangle has a different base while the remaining two side-lengths are equal, therefore:
x + 18 = 3x
Subtract x from both sides:
18 = 2x
Divide both sides by 2:
9 = x
Therefore, x = 9.
