Suppose $f$ is an even function and ${\displaystyle \underset{-7}{\overset{7}{}}}f(x)dx=10.$

a$.$ Evaluate ${\displaystyle \underset{0}{\overset{7}{}}}f(x)dx$

b$.$ Evaluate ${\displaystyle \underset{-7}{\overset{\phantom{}}{}}}xf(x)dx$

a$.$ Evaluate the definite integral.

${\displaystyle \underset{0}{\overset{7}{}}}f(x)dx=,(Simplify$ your answer.$)$