First of all, since different lengths appear in the task, we will establish a relationship between these values:
1 cm = 10 mm.
Thus, taking into account this ratio, we translate the length of the longer side into millimeters:
7 cm = 7 * 10 mm = 70 mm.
Now you can calculate the perimeter of the rectangle. Let’s define it by the following formula:
P = 2 * (a + b).
Let’s apply this formula:
P = 2 * (70 + 3) = 2 * 73 = 146 (mm).
If you need an answer in cm, just divide it by 10 (we get 14.6 cm).
Answer: 146 mm.
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