I think you meant to have more of a problem stated.
Basically, you use the Law of Sines when you have 2 angles but the length of only one side.
The formula for this law is
a / sine(A) = b / sine(B) = c / sine(C)
The sequence is increasing by the previous number multiplied by negative 3 (x -3). With this information, the correct answer would be A. -648, 1944, 5832

This result is actually true for any exterior angle. The exterior angle of a triangle is equal to the sum of the two remote angles, and above is a short proof of it.
Rational numbers are numbers that can be turned into fractions. So A, C, D, E, F, G, H, and I are rational numbers
Here is what to do:
Add like terms. For example, you can add apples to apples, but not apples to oranges. In other words, you can add similar values of x with its others, but not the values of x with regular numbers.
The answer is 3x-3