Skill: Solve one-step subtraction equations

A subtraction equation is a statement that both sides have the same value. To solve it, use addition to undo subtraction while doing the same thing to both sides.

First notice where the variable is:

• In $x-a=b$, add $a$ to both sides.
• In $a-x=b$, the variable is being subtracted, so first isolate $-x$.

Example: Solve

$$8-x=13$$

Since $x$ is the second number being subtracted, do not simply add $8$. First remove the $8$ by adding $-8$ to both sides:

$$8-x-8=13-8$$

This simplifies to

$$-x=5$$

Now take the additive inverse of both sides to change $-x$ into $x$:

$$x=-5$$

Check the solution by substituting $-5$ for $x$:

$$8-(-5)=8+5=13$$

The equation is true, so the solution is

$$\boxed{x=-5}$$