First, we have to take the area of the toy store.
3m*3m= 9 sq. m

Since it's said that she has 55 sq. m left on her house, we will add our toy store area to the area left in order for us to get the total area of her lot.
55 sq. m + 9 sq. m = 64 sq. m

Now that we know the area, the only thing left is to take the dimension from that.
Since it is a square lot, we just simply take the square root of the area to get the values of each side of the lot.
 \sqrt{64 m^2} =8m

Therefore, the original dimension is 8m x 8m.

Hope that helps!