Note that a square has the same measurements for each of it's sides.
The area for side is:
Area = side x side.
let side = s, Area = a
a = s²
Plug in 121 for a
121 = s²
Isolate the s. Note the equal sign. What you do to one side, you do to the other. Root both sides
√121 = √s²
s = √121 = √11 x √11
Note that two of the same roots when multiplied will equal the number as a whole number
s = √11 x √11
s = 11
11 in. is your answer
hope this helps