Question 100904
Lets call the width x
Now define the length in terms of x
Given: the length is three times as long as the width
length = 3x
Given: area of rectangle is 108 square millimeters
Now since we know that the area of a rectangle equals length times width
we can write a formula to solve for x
(length)(width) = area
{{{(3x)(x)=108}}}  multiply terms
{{{3x^2=108}}}     divide both sides by 3
{{{x^2=36}}}     take square root of both sides
{{{x=6}}}        
<b>Answer: width is 6 millimeters</b>
Next we can find the length
length =  3x
length = 3(6)
length = 18
<b>Answer: length is 18 millimeters</b>
Check answers with the formula for area
length times width = area
(18)(6)=108
108=108