$\begin{array}{l} 6\sqrt {36{a^2}} - 6a\\ = 6\left| {6a} \right| - 6a\\ = 6\left( { - 6a} \right) - 6a = - 42a\left( {a < 0} \right)\\ \sqrt {121{a^2}} + 3a\\ = \sqrt {{{\left( {11a} \right)}^2}} + 3a\\ = \left| {11a} \right| + 3a\\ = 11a + 3a = 14a\left( {a > 0} \right) \end{array}$