According to the problem, Heather already knew 19 recipes.
She will learn at a rate of 3 recipes per week.
To know the total number of weeks it will take to know 28 recipes, we form the following equation.
![y=19+3x](https://img.qammunity.org/2023/formulas/mathematics/college/pzltoumgishn3leapquw4cs7xz6a1e51c4.png)
Where x represents weeks, and y represents recipes. Let's solve for x when y = 28.
![\begin{gathered} 28-19=3x \\ 9=3x \\ x=(9)/(3) \\ x=3 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/3ubb19t65y8tpa4yh8qp5tgdgbuyveiir3.png)
Hence, it will take 3 weeks only.