29.) This is simply interpreting the linear function.
The function:
96+2.1x
a.) The 96 is what you start with from the beginning. So, we know Rachel weighed 96 ounces when she was born.
b.) Again, interpretation. The 2.1 is the slope, or the change in the line. So we know that Rachel gains 2.1 ounces each day.
30.) For this one, you just have to plug 9 into the equation for x.
