Reading the solution, please, look on the picture)
Let the side of the square will be X meters. This means that S1=x^2 meters ^2
One side of new rectangular garden will be 2x meters and another (x-3) meters. So S2=2x(x-3) meters ^2
According to the data:
S1*1.25=S2
x^2*1.25=2x(x-3)
1.25*x^2=2x^2-6x
0.75x^2-6x=0
3x^2-24x=0
x^2-8x=0
x(x-8)=0
x=8 or x=0
But x>0!
So x = 8 meters
The next question is to find S2.
S2=2x(x-3)=2*8*5=80 meters ^ 2