Brenna is making brownies. She wants to make an extra pan for her grandparents, so she is doubling the recipe. If the original recipe calls for n units of an ingredient, she uses 2n units for her doubled recipe. The original recipe calls for 1.5 teaspoons of vanilla extract.
Well, she’s using double the amount of ingredients so you need to double 1.5. 1.5 x 2 = 3. She needs 3 teaspoons of vanilla extract.