Try the following GMAT Problem Solving question on conditions under which algebraic expressions are undefined.
Question 51:
For what real values of $x$ is the expression $\dfrac{3(x+3)}{(x-1)(x+2)}$ not defined ?
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$\quad x=-3 \; \textrm{only}$
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$\quad x=-2 \; \textrm{only}$
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$\quad x=1 \; \textrm{only}$
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$\quad x=-2 \; \textrm{or} \; 1 \; \textrm{only}$
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$\quad x=-3, -2, \textrm{or} \; 1 $