A.
Explanation:
There are several reasons that Carbon-12 was chosen as the standard for determining the relative mass of other atoms.
- Its abundance on earth
- Carbon-12 is a very stable isotope atom
- Its has the same number of neutrons and protons hence more ideal to use for a precise calculation of <em>amu</em> (atomic mass unit) -
The mass of an atom are based on the ¹/12 mass of Carbon-12.
In favour, if i understand the question correctly
3x + 7 = x - 15
First, make sure to subtract

from both sides.
Second, simplify 3x - x to get 2x, and then keep the 7.
Third, subtract

from both sides.
Fourth, simplify -15 - 7 to get -22.
Fifth, take the 2 and divide both sides by it, keeping the

by itself.
Sixth, since 11 goes into 2 to get 22, simplify the fraction to 11.

Answer:
x = -11
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