Hi there!
Since we know that the formula for the area of a rectangle is A = l x w, we can plug in what we know and go from there.
WORK:
10.5 = 3.5 x w (plugging in values)
10.5 = 3.5w (simplifying)
w = 3 feet (dividing)
Hope this helps!! :)
If there's anything else that I can help you with, please let me know!