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

hard

$ax-by=72$
$2ax-8y=48$

In the given system of equations, $a$ and $b$ are constants. The graphs of these equations in the $xy$-plane intersect at the point $(x,6)$. What is the value of $b$?

A

$-8$

B

$-4$

C

$4$

D

$6$