When x Is Negative

In 10 - x = 2x + 1, the x on the left has coefficient -1. To collect toward the larger coefficient, move it to the right: add x to both sides.

10 = 3x + 1

9 = 3x

3 = x

Swap the sides to write it in the usual form, x = 3. Check: 10 - 3 = 7 and 2 × 3 + 1 = 7.

Adding an x-term to both sides is allowed in the same way as subtracting one. It is the step to use whenever an x-term has a minus sign in front of it. You could instead subtract 2x from both sides and get 10 - 3x = 1. That also works, but the coefficient stays negative until the end.