That appears to be Tithonia rotundifolia, also known as Mexican Sunflower. Note the rounded petals and similar leaves to your plant on this photo. It's an annual in non-tropical areas but has naturalized in areas with warm winters. The species can reach the heights you've described, while the 'Torch' cultivar is smaller at only 1m or so in height and width. If you'd like more of this plant next year, you can save the seeds if you want: start them indoors in very early spring (under lights, if possible) - maybe 4-6 weeks before your typical last frost date - and then plant out after the last frost in your area.