Hey there! I'm happy to help!
Since this mural is a square, all the sides are the same. This means that you square on of the sides to get the area (multiply it by itself). If we have the area and want to find a side, we just go backwards and use something called the square root!
If we plug this into our calculators...
√138≈11.74734
We see that is closest to B. 12 ft.
You could also figure this out without a calculator. 138 is very close to 144, which is 12². All the other answer options are way too big.
Have a wonderful day! :D
