Answer:
x = 100 meters
Explanation:
The area of the original plot is 200 x 300 = 60,000
Doubling the area would equal to 120,000
(x + 200) x (x + 300) = 120,000
x^2 + 500x + 60000 = 120,000
x^2 + 500x - 60,000 = 0
(x + 600)(x - 100) = 0, so
x = - 600 (reject), x = 100
Therefore x = 100 meters.
I hope I could help :)