You need to use simplification. What you do to one side of the equation, you do to the other. I'll use your number 2 as an example.
3x + 5 = 2x + 13
We want to get x by itself on one side, so to start lets subtract 2x from both sides:
3x-2x + 5 = 2x - 2x + 13
x+5 = 13
now, we subtract 5 from each side to get x by itself:
x+5-5 = 13-5