The method for these really aren't that hard, let's take one of your examples:
1. Split up all like terms
We can see that 2 and 1 are just regular numbers and that 7x and -7x have an x in common, so we will split them up that way, I like to use parentheses to avoid confusion:
2. Solve the parentheses 1 by 1.
We can see that 2 + 1 is three, but the 7x - 7x is tricky. For a moment, let's pretend that there is not an x there, we would then have:
This is obviously 0.
Now, let's put the x back in, this time we multiply everything by x, even the zero, since they are like terms:
(Zero times anything is zero)
And there you have it, we just put that where we put the (7x - 7x) in the problem:
Hope this helps.