Question 722966
<pre>
If she has studied imaginary numbers she can factor it,
since i² = -1  and therefore
     -i² = 1

16x² + 81 =

16x² + 81·1

Now substitute -i² for the 1

16x² + 81·(-i²)

16x² - 81i²

(4x - 9i)(4x + 9i)

However if she has not studied imaginary numbers, then she
should just write "prime". 

Edwin</pre>