\[\ a < 0:\]
\[7a\sqrt{\frac{16}{a^{2}}} = \frac{7a \cdot 4}{|a|} = \frac{28a}{- a} = - 28.\ \]
\[6,1^{2} = 37,21;\]
\[6,2^{2} = 38,44;\]
\[6,1 < \sqrt{38} < 6,2.\]