I mean, I don't know how to explain a generic way to solve this. Ask me about the specific train of thought if you are interested:
7 * 18 + 45 / 3 - 2 = 139
Subtract 9 from both sides of the equation
3x + 7 = -x - 1
3x + 7 + = -1
4x + 7 = -1
4x = -1 - 7
4x = -8
x = -2
1 - 2x + 5 = 4x - 3
-2x + 6 = 4x - 3
6 = 4x - 3 + 2x
6 = 6x - 3
6 + 3 = 6x
9 = 6x
x = 6/9
x = 2/3
i helped you with the first two questions so you can see how you can solve it.
hope this helps, God bless!