Question 776872
the longest side is 21 and the ratio of all the sides are 3;5;7

fortunately this one should be simple to figure out, 21 can be divided by 7 three times, so just go ahead and multiply the other ratio numbers by 3 to get your side lengths

3;5;7,    9, 15, and 21   add those 3 up to get your perimeter of 45