A 30-60-90 triangle follows a unique pattern. It's hypotenuse is double the shortest leg and its longest leg is the shortest leg times the square root of 3.
In this case, we are given the hypotenuse. The hypotenuse is 2 times the shortest leg, so the shortest leg, f, must be 10.
Then, the longest leg of the triangle is the shortest leg times the square root of 3. Therefore, the longest leg is 10 times sqrt(3).
Hope this helps!
