Answer:
<em>The length of the side along the river is 140 m</em>
Step-by-step explanation:
<u>Equations</u>
To solve the problem, we need to recall the area of a trapezium is:

Where b1 and b2 are the lengths of the parallel sides and h is the height of the trapezium.
The field to be bought by Mohan has a trapezium shape with a side along the road of side x and a side along the river of side 2x, as stated in the problem.
Knowing the height is h=100 m, and the area is 10500 m2:

Simplifying:


Dividing by 150:

x = 70 m
2x = 140 m
The length of the side along the river is 140 m