Question 497901
Using the vertex at the top and the longest side as the base, you can drop the altitude of the triangle. This will split the isosceles triangle into two right triangles. Each congruent side is the hypotenuse. 
Knowing the height and using 1/2 the base, we can use the Pythagorean theorem to determine the hypotenuse. 
11^2 + 18^2 = c^2
121 + 324 = 445.
{{{sqrt(445)}}} = hypotenuse.
Approx. 21.1  = hypotenuse.
This is the measure of the other two sides.
Cleomenius.