You can put this solution on YOUR website! 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
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