[Image of 4 integers A, B, C, and D on a number line]

The integers A, B, C, and D shown on the number line above are all equally spaced. If C and D are equal to 5^12 and 5^13, respectively, what is the value of A?

- 1. 5^11

2. 5^10

3. -5^12

4. (-7)5^12

5. (-12)5^13

I can set up the equation: [spoiler]D minus three times the difference between C and D (or) 5^13 - 3(5^13-5^12) = A.[/spoiler] But after that, I'm lost with the algebra.

Below is the algebra for the right answer:

[spoiler]5^13 - 3(5^13 - 5^12)

5^13 - 3[5^12(5 - 1)]

5^13 - 3(5^12)(4)

5(5^12)- 12(5^12)

(-7)5^12

The correct answer is D.[/spoiler]

Can someone explain what operations are taking place in the first 2 lines?

Thanks in advance.