The answer you would be looking for is A.12.4 feet. You can use the equation A^2+b^2=c^2 to solve for your x being length.
Making 4^2+b^2=13^2 or
16+b=169
Take 16 from both sides
B=153 then square root both sides
B^2 or x= 12.36 or 12.4 if you choose to round up as asked in the question.
I hope this helps!
