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Answer:
transitive property of equality
Explanation:
The transitive property of equality says ...
if a = b and b = c, then a = c.
Here, we can use this form with ...
a = 2x+4
b = y
c = 3x-6
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We have ...
a = b ⇒ 2x +4 = y
b = c ⇒ y = 3x -6
so, ...
a = c ⇒ 2x +4 = 3x -6 . . . . transitive property of equality.
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On the other hand, ...
The substitution property of equality says ...
if a = b, we can use b wherever 'a' is seen.
We can use this with ...
a = y
b = 3x -6
and do the substitution in the equation ...
2x +4 = y
Replacing y with its equal, we get ...
2x +4 = 'a' ⇒ 2x +4 = 'b' ⇒ 2x +4 = 3x -6 . . . . substitution property of equality
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Additional comment
These are two different ways to say that anything can be replaced by its equal.