Given that, the length of the garden is 2(x+6)feet and width is 3.5x feet.
So, length : l = 2(x+6) and width = 3.5x.
To find the fence of the garden we need to find the perimeter of the rectangle.
Formula to find the perimeter is:
p = 2l + 2b
= 2*2(x+6) + 2*3.5x
= 4(x+6) + 7x By multiplication.
= 4x + 24 + 7x By distribution property.
= 11x + 24 Combining the like terms.
So, he need to buy 11x + 24 feet fencing.
Hope this helps you!