Answer:
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This is the question that's gonna put me over the top and make me a Brainly Genius. Of course working for free doing middle school math homework doesn't really sound like a genius, but I'm doing it anyway.
This one is the Pythagorean Theorem of course:



The Shakespeare Quadratic Formula (2b or -2b) is a little shortcut for the quadratic formula when the middle term is even.

So we get

The minus root is less than -6 giving negative sides, so we reject it and conclude

Choice C
When I hit Add I officially become a Genius. Maybe I should retire.
-4x + y = 6
-5x - y = 21
add both of them
-9x = 27
x = -3
(-4*-3) + y = 6
-12 + y = 6
y = 18