AThis triangle is half of an equilateral triangle because the angle of an equilateral triangle is 60°. The height of the equilateral triangle of side 18 is found using Pythagras’ Theorem, by splitting the triangle into two with a perpendicular bisector which gives us two back to back triangles the same as the triangle described in the question. So we have half the length of the side as the base, 18/2=9. And the height²+9²=18².
height²=18²-9²=324-81=243 and height=√243=9√3=15.5885 approx.
Area of the given triangle = half base times height = ½×9×15.5885=70.1481 approx. square units.