Hi Tyler
Let's see what we have.
For the garden
L = length
W = width
W = (2/3)L
Fencing = 300'
To enclose a garden we need to find the perimeter of the garden.
P = perimeter
P = 2(L+W)
Since W = (2/3)L, then
P = 2[L+(2/3)L] =300
Let's rewrite this so it's clear
2L + 4/3 L = 300
10/3 L = 300
L = 90
Since W = 2/3L, then
W = 60
And to double check
2(L+W) = 2(60+90) = 2(150) = 300
The dimensions of the garden are 90' by 60'