$\displaystyle{{11}}^{{2}}={121}\usepackage{cmbright}\Huge\$ , which is less than 136, and $\displaystyle{{12}}^{{2}}={144}\usepackage{cmbright}\Huge\$ , which is greater than 136. The square root of 136 must be between 11 and 12, but there are no whole numbers between 11 and 12, so 136 is not a perfect square.