Find the image of the triangle ABC under the homothety with center O and ratio k=-2. Draw rays from points A, B, C through point O. Find with compass point A' that line segment OA' is two times longer as OA. The ratio k is negative then A' is on different side of the point O. In the same way find point B' and C'. Join points A', B' and C'. We get the image of the triangle ABC under the homothety with center O and ratio k=-2.