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.
[tex]y=19+3x[/tex]Where x represents weeks, and y represents recipes. Let's solve for x when y = 28.
[tex]\begin{gathered} 28-19=3x \\ 9=3x \\ x=\frac{9}{3} \\ x=3 \end{gathered}[/tex]