As Moo of Doom said, it's not clear what your question is. For one thing you ask for "x" but the only "x" in your question is the one you used as a multiplication sign!
If you asking how to convert 600 square feet into square meters, then using arildno's method, you have to know that there are approximately .3048 meters per foot. Therefore, there are approximately(.3048)2(600)= 55.7 square meters in 600 square feet.
Or perhaps you are asking how to convert 200 square meters into square feet. Since now you are going the "opposite way", divide by .30482 instead of multiplying. There are approximately 200/(.3048)2= 2152.8 square feet in 200 square meters.
Or you might be asking for length and width of a rectangle such that
length times width= 200 m. Unfortunately there are many different answers to that even if you require integer length sides: for example, 1 m x 200 m= 200 square meters, 2m x 100 m= 200 m, 4 m x 50 m= 200 square meters, etc. If you allow fractional lengths, there are an infinite number of solutions. (And it has nothing to do with converting from feet to meters.)
Perhaps you are asking a somewhat more complicated question: what must be the length and width of a rectangle of the same proportions as 20' x 30' which has area 200 square meters. In that case, if we let w= width, h= height, since the 20' x 30' rectangle has ratio, height to width, of 30'/20'= 3/2, we must have h= (3/2)w. Since area= hw= 200,
(3/2)w2= 200, w2= (2/3)(200)= 400/3 and
w= sqrt(400/3)= 20/sqrt(3)= 11.5 meters and h= (3/2)(11.5)= 17.3 m approximately.