Let

${\displaystyle \underset{0}{\overset{2}{}}}f(x)dx=5,{\displaystyle \underset{0}{\overset{3}{}}}f(x)dx=15,{\displaystyle \underset{0}{\overset{2}{}}}g(x)dx=-13,{\displaystyle \underset{2}{\overset{3}{}}}g(x)dx=-7$

Use these values to evaluate the given definite integrals.

$($A$){\displaystyle \underset{0}{\overset{2}{}}}(f(x)+g(x))dx=$

$($B$){\displaystyle \underset{0}{\overset{3}{}}}(f(x)-g(x))dx=$

$($C$){\displaystyle \underset{2}{\overset{3}{}}}(3f(x)+2g(x))dx=$

$($D$)$ Find the value a such that ${\displaystyle \underset{0}{\overset{3}{}}}(af(x)+g(x))dx=0.$

$a=$