Consider two functions $f$ and $g$ on $1,7$ such that ${\displaystyle \underset{1}{\overset{7}{}}}f(x)dx=12,{\displaystyle \underset{1}{\overset{7}{}}}g(x)dx=5,{\displaystyle \underset{6}{\overset{7}{}}}f(x)dx=5,$ and ${\displaystyle \underset{1}{\overset{6}{}}}g(x)dx=2.$ Evaluate the following integrals.

a$.{\displaystyle \underset{1}{\overset{6}{}}}5f(x)dx=($Simplify your answer.$)$