Okay, so your scale rectangle has a length of l = 8.2 cm. The actual triangle has a length of L = 41 cm. What is the scale factor? You find out by dividing the scale rectangle l from the actual rectangle L.
41 cm / 8.2 cm = 5
The scale factor is 5. That means the actual rectangle is exactly 5 times bigger than the scale rectangle. Now finding the width is easy! If the width of the scale rectangle is 2.1 cm and the scale factor is 5, you just multiply them together!
2.1 cmx 5 = 10.5 cm
The width of the actual rectangle is 10.5 cm! Hope that helps!