The equation of the area of a triangle is Area=Length×Width. We know area=256.5 and length is 18. So now we know 256.5=18×width. We then divide 256.5÷18 to get the width. The width is 14.25. Perimeter=legth+length+width+width. If width is 18 and length is 14.25 then we add up all 4 sides. 18+18+14.25+14.25=64.5. 64.5 is the perimeter