Let's set it up to make it easier:
81 = 9
? = ?
121 = 11.
When taking note of perfect squares and their roots, they tend to be written in numeric order (1,2,3...). We know that the number 10 is in between 9 and 11, so we need to find out the square of 10.
10^2 (or 10*10 )= 100. 100 = 10.
