Question 69586
To find the perimeter (P) of a rectangle you need to add 2 times the length (L) to 2 times the width (W) of the rectangle.  In equation form this is written:

{{{2L + 2W = P}}}

You are given that P is 47mm and L is 18.5mm.  Substitute these values into the equation to get:

{{{2*18.5 + 2W = 47}}}

Multiply out the 2*18.5 and the equation becomes:

{{{37 + 2W = 47}}}

Subtract 37 from both sides to remove it from the left side of the equation.  The result is:

{{{2W = 10}}}

Finally, divide both sides by 2 and you find that:

W = 5 mm.

Then recognize that the area of a rectangle is found by multiplying its length times its width.  You were given that the length was 18.5 mm and you found that the width was 5 mm.  Multiplying these two together results in an area (A) equal to:

{{{A = 18.5*5}}}

which multiplies out to:

{{{A = 92.5}}} 

and the units of the answer are square mm