One step addition and subtraction

A one-step equation is undone by doing the opposite operation to both sides.
Addition is undone by subtraction, and subtraction is undone by addition.
Doing it to both sides keeps the equation balanced.

If a number is added, subtract it.
From $a + 15 = -10$, subtracting $15$ from both sides gives $a = -10 - 15 = -25$.
If a number is subtracted, add it, as in $x - 6 = 12$, where adding $6$ gives $x = 18$.

The same one step can isolate a whole term or a quantity in a formula.
From $3x + 2y = 12$, subtracting $2y$ from both sides gives $3x = 12 - 2y$.
From $a + b = cd$, subtracting $a$ gives $b = cd - a$.

Whatever you add or subtract must be done to both sides.
Doing it to only one side would change the equation.
On the side with the variable, the added and subtracted amounts cancel, leaving the variable by itself.

Solve $x - 6 = 12$.
Add $6$ to both sides: $x - 6 + 6 = 12 + 6$.
So $x = 18$.

Solve $a + 9 = -4$.
Subtract $9$ from both sides: $a = -4 - 9$.
So $a = -13$.

Given $a + b = cd$, solve for $b$.
Subtract $a$ from both sides.
So $b = cd - a$.