Hey there! :)
If the width is 10 ft and the farmer wants the length to be 3x the width:
Length = 3w
W = 10
10 × 3 = 30
The width is 10 ft and the length is 30 ft.
Remember: You have to find the perimeter
Perimeter = 2l + 2w
l = 30
30 × 2 = 60
w = 10
10 × 2 = 20
60 + 20 = 80
Your answer is 80 ft of fencing
Hope this helps :)