This problem requires the pathagoriun theorum. The formula is a² + b² = c², which basically means that If you square the two sides that are already labeled, then add them, you will get the length of the side that is unknown.
In this equation, we are given the length of the shortest side, which is 21m. I believe that the length of the side CA is twice the length of the side BC, which would be 42m.
Now you find the square of both, and add them. 21² + 42² (or 441 + 1764) is 2205.
Since the square of 2205 is a decimal, (it is 46.9574275275) you will round it to the nearest whole number, which is 47.
