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Math·Algebra·Systems of two linear equations in two variables

hard

$$\{\begin{array}{l}x+3y=-9\\ 3x+4y=13\end{array}$$
The solution to the given system of equations is $(x,y)$. What is the value of $x+y$?

A

$-9$

B

$-7$

C

$7$

D

$9$