1. Let's clarify why we set $a-2=0$ in some problems. 2. Usually, this happens when we want to find the value of $a$ that makes an expression equal to zero or satisfies a certain condition. 3. For example, if you have an equation like $a-2=0$, it means we want to find $a$ such that the expression equals zero. 4. To solve it, add 2 to both sides: $$a-2=0 \implies a=2$$ 5. So, $a$ must be 2 to satisfy the equation. 6. This is a common step in algebra to find unknown values that make an equation true.