<PRE><B>Question 913435</B>
So you know the width is 3cm and the length is 9cm. When you draw a diagonal from one corner of the rectangle to its opposite corner, you create a right triangle. To find the length of that diagonal (which is the hypotenuse) you have to use the Pythagorean theorem:
a^2 + b^2 = c^2
So now you substitute:
3^2 + 9^2 = c^2 
9 + 81= c^2
c^2 = 90
c = sqrt(90) 
c = 9.486
You can also round your answer to:
c = 9.5cm </PRE>