1. The problem asks to find the value that makes an expression undefined. 2. Typically, expressions become undefined when there is division by zero or taking the square root of a negative number (in real numbers). 3. Since the exact expression is not given, the most common cause is division by zero. 4. For a fraction \( \frac{a}{b} \), the expression is undefined when \( b = 0 \). 5. Therefore, to find the value that makes the expression undefined, set the denominator equal to zero and solve for the variable. 6. For example, if the expression is \( \frac{1}{x-3} \), set \( x-3=0 \) which gives \( x=3 \). 7. Hence, the expression is undefined at \( x=3 \). 8. Without the specific expression, the general rule is: find values that make the denominator zero or cause invalid operations.