Which of the following equations has infinitely many solutions?
3x + 5(2x + 6) = 13x + 30
5x + 2 = 2x + 6
3(x + 5) + 2 = 6x + 3(x + 2)
3x + 5 = 2

For an equation to have an infinite possible solutions, both sides of the equal sign, must be completely equal. (Or something very very close)

You can scratch the bottom and the second one, just by looking at it.

After that, we need to simplify. 

Simplifying the first:

3x + 10x + 30 = 13x + 30
13x + 30 = 13x + 30

Oioioi seems like we got lucky :)

Both sides match, so it dosent matter what value x is, it will still match.
13*5 + 30 = 13*5 + 30
65 + 30 = 65 + 30
95 = 95

Understand?

Can simplify the other one, if needed. But it should not be necisarry in this context.