Question 159213
{{{Area = Length * Width}}}
You are told the area is 210.
{{{210 = L*W}}}

You are also told one side of the lot doesn't have a fence; it has a river.

The Fencing is 42.

Normally the perimeter of a rectangle is given by {{{Perimeter = 2*Length + 2*Width}}}

However, in this case, one of the length sides is on the river. So, the total fencing perimeter is given by {{{FencingPerimeter = 1*Length + 2*Width}}}
{{{42 = L + 2W}}}
{{{42 - 2W = L}}}

Substitute this value for L back into the Area equation
{{{210 = L*W}}}
{{{210 = (42-2W) * W}}}
{{{210 = -2W^2 + 42W}}}
{{{0 = -2W^2 + 42W - 210}}}
{{{0 = W^2 - 21W + 105}}}

Use the quadratic equation to solve
*[invoke quadratic "x", 1, -21, 105]

So you have two possible answers for the width. Width is either 8.2 or 12.8
Use the Perimeter equation to find values for Length at each of these values fro width.
If width = 8.2, then {{{Length = 42 - 2*8.2}}} = {{{25.6}}}
If width = 12.8, then  {{{Length = 42 - 2*12.8}}} = {{{16.4}}}

Since the Length in both cases is longer than the width, you lot can be either 
16.4 by 12.8 or 25.6 by 8.2 meters