Collect the variable and numbers separately: add x to each side: -29=13+3x; then subtract 13 from each side: -42=3x which is the same as 3x=-42. Divide through by 3: x=-14. You can see how the equals sign has been used to separate the knowns (numbers) from the unknowns (x's). That's the clue for all problems like this one: separate knowns and unknowns by driving (negotiating) the equals sign between them.