#### Answer

$23m\sqrt {n^{5}}$

#### Work Step by Step

Step 1: $5\sqrt {49m^{2}n^{5}}-3m\sqrt {16n^{5}}$
Step 2: $5(\sqrt {49m^{2}}\times\sqrt {n^{5}})-3m(\sqrt {16}\times\sqrt {n^{5}})$
Step 3: $5(7m\times\sqrt {n^{5}})-3m(4\times\sqrt {n^{5}})$
Step 4: $35m\sqrt {n^{5}}-12m\sqrt {n^{5}}$
Step 5: $\sqrt {n^{5}}(35m-12m)$
Step 6: $\sqrt {n^{5}}(23m)$
Step 7: $23m\sqrt {n^{5}}$