Explanation:
21m
Let the width of the rectangle be x.
Then,the length of the rectangle = 3x
As, perimeter of the rectangle = 56m
2 × (Length + Breadth) = 56
2 × (3x + x) = 56
2 × 4x = 56
8x = 56
\(\Rightarrow\text{x}=\frac{56}8{}\)
\(\therefore\text{x}=7\)
So, the length of the rectangle = 3x = 3 × 7 = 21m.
Hence, the correct alternative is option (c).