The green line labeled D is the diagonal.  Let's erase the left 
and top sides of the rectangle, like this:



Now as you see, we only have a right triangle and its hypotenuse
is D.  So we use the Pythagorean theorem:

D² = 8² + 4²
D² = 64 + 16
D² = 80
 D = 

You can either do that with a calculator and get

D = 8.94427191 cm., rounding it 
off as your teacher requires.

Or else you can give the exact answer in radical form:

D =  cm 

Edwin