Answer:
525 x 1,050
A = 551,250 m²
Step-by-step explanation:
Let 'L' be the length parallel to the river and 'S' be the length of each of the other two sides.
The length of  the three sides is given by:

The area of the rectangular plot is given by:

The value of 'S' for which the area's derivate is zero, yields the maximum total area:

Solving for 'L':

The largest area enclosed is given by dimension of 525 m x 1,050 and is:

 
        
             
        
        
        
Exchange : 63m = 6300 cm; 56m21cm= 5621cm
So the lengths of the ribbon will be left is : 6300 - 5621 = 679cm = 6.79 m 
Hope it helps! If it is, Brainliest please!
        
             
        
        
        
3x + 6 = 12
First operation was subtraction (subtracting 6 to leave 3x alone)
Second operation was division (Dividing 3 from 3x to isolare x)
3x + 6 = 12
3x + 6 -(6) = 12 - 6
3x = 6

 = 

3 and 3 cancels out
x = 2