let
x--------> the number
we know that
11²=121
12²=144
then
x² must be greater than 121 and less than 144
case a) √115
if x=√115
then
x²=115-------> is not greater than 121
case b) √121
if x=√121
then
x²=121-------> is not greater than 121
case c) √136
if x=√136
then
x²=136-------> is greater than 121 and is less than 144------> is ok
case d) √150
if x=√150
then
x²=150------> is not less than 144
therefore
the answer is
√136
