Answer:
Which equation is true when the value of x is -12 HERE YOU GO!!
Step-by-step explanation:
tricky ... let's see ...
I notice that if we subtract xy from both sides we get
7x + xy - xy = xy - xy + 21
then
7x = 21
and
x = 21/7 = 3
so there is only one value of x that satisfies the equation
x = 3
Going back to the original equation we see that any value of y will satisfy the original equation
we can see this by rearranging things:
7x + xy - xy = 21
here, I have performed the subtraction of xy on the right side as above, but have left the left side undone
(so we don't lose the presence of y)
Note that the above can also be written as
7x + (x - x)y = 21
or
7x + 0y =21
now, since anything times zero equals zero,
y may be any number.
Let's summarize:
1) x = 3
2) y = anything
looking back at the original question;
1) the equation is true for all ordered pairs
FALSE (only one x works, not all x)
2) there are no x and y pairs for which the equation is true.
FALSE (x=3, y=anything) makes it true, i.e. (3,1)
3) For each value of x, there is one and only one value of y that makes the equation true,
FALSE for each value of x, for the one value of x, x=3, y can be any number, which is an infinite number, not one
4) for each value of y, there is one and only on value of x that makes the equation true.
TRUE!! for all the infinite values of y you may pick, there is one and only one x you may pick, x = 3
ANSWER: STATEMENT 4 is CORRECT (TRUE)
. We will fill in each area and then solve for the radius. 
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so r = 25. And there you have it!