1. The problem asks to list variables and constants in each expression. 2. Variables are symbols that can change values; constants are fixed values. 3. For expression 1: $3x - 6$ - Variable: $x$ - Constant: $3$, $6$ 4. For expression 2: $3ab$ - Variables: $a$, $b$ - Constant: $3$ 5. For expression 3: $\pi r$ - Variable: $r$ - Constant: $\pi$ (pi is a mathematical constant approximately 3.14159) This completes the first question as per instructions.