Hello there! It should take her 8.6 weeks.
Well, the question doesn't specify whether it wants how long it'll take her in weeks, months, or days, so let's go with weeks. To do this, find the unit rate by dividing the amount she lost by the number of weeks it took.
3.5 pounds ÷ 2 weeks = 1.75 pounds lost a week.
Now, divide the total she wants to loose (15 pounds) by the 1.75 pounds a week to see how many she looses in total.
15/1.75 = 8.57142... Which is a little over 8.5 weeks, so we can round up to 8.6 weeks.