18)

This is an isosceles triangle. The altitude to the base bisects the base. The base is $$16$$ cm. The altitude divides the triangle into two right-angled isosceles triangles. $$y$$ is the altitude. The hypotenuse of each small triangle is one of the equal sides of the large isosceles triangle. The base of each small triangle is $$16/2 = 8$$ cm. Since it's an isosceles right triangle, $$y = 8$$ cm.