Solution :-
We know that perimeter is the sum of all sides, and here we have been provided with perimeter and the length of three sides. Now, we can assume the length of other side as any variable, say x. And develop a linear equation, and solve it. Thus,
A/Q,
→ 3yd + 6yd + 9yd + x = 24yd
→ 18yd + x = 24yd
→ x = 24yd - 18yd
→ x = 6yd
Thus, the length of the fourth side is 6yd.
Hope that helps :)