First find the side length by dividing the perimeter by 3: 13.5/3=4.5cm.
Draw a line length 4.5 cm.
Using compasses set to radius 4.5cm and centre point at one end of the line, draw an arc which is roughly over the midpoint of the line, or draw a complete circle. Do the same at the other end of the line so as to cut the arc or circle. Join this intersection to each end of the line to produce the equilateral triangle.