Try this option:
note, that distance between the parallel sides is shown via formula of area.
If it is possible, check the arithmetic.
Answer:
18
Step-by-step explanation:
Just divide both sides by - 64
Perimeter of Rectangle: 2W + 2L
One L is the river so we have 3 sides
420m/3 = 140m
Area of Rectangle is WL
140×140= 19,600 Sq meters
I'm assuming you may have another mathematical system the teacher wants you to use. Yet, I get Area is 19,600 Sq meters
Treat it just like and equation at first. Multiply the 10 on both sides to get rid of it, leaving you with 50 
x. Then on the number line, since there is a line under it, you would make a filled in circle and an arrow pointing towards 51 and beyond
I think your best answer is a pasture that is 75 feet by 150 feet, which will give you 11,250 sq ft of pasture
