To solve this problem you must apply the proccedure shown below:
 1- You have that the laddder makes an angle of 
 with the ground and the top of the ladder reaches
 with the ground and the top of the ladder reaches 
 .
.
 2- Therefore, you have:
 

 Where:
 

 3-You must substitute values and solve for the lenght, as following:
 

 Therefore, the answer is: 
