What is the length of a portion BC of the cloth

Third option.

Step-by-step explanation:

You must analize the information given in the exercise.

By definition Right triangles are those triangles that have an angle that measures 90 degrees.

In this case you can see in the picture attached in the exercise that there is a Right triangle ABC.

 Then, in order to solve the problem you need to apply the following Trigonometric Identity:

You can identify in the Righ triangle ABC that:

Therefore, knowing those values, the next step you must follow is to make the substitution into  , as below:

Finally, solving for BC, you get that:

if you was gonna solve for a, a is a   < -12