Consider two functions $f$ and $g$ on $1,8$ such that ${\displaystyle \underset{1}{\overset{8}{}}}f(x)dx=13,{\displaystyle \underset{1}{\overset{8}{}}}g(x)dx=6,{\displaystyle \underset{5}{\overset{8}{}}}f(x)dx=5,$ and ${\displaystyle \underset{1}{\overset{5}{}}}g(x)dx=2.$ Evaluate the following integrals.

a$.{\displaystyle \underset{1}{\overset{5}{}}}7f(x)dx=($Simplify your answer.$)$